diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3eb5aaee..652b995a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,11 @@
 
 ## [3.x - Unreleased] - 2022-xx-xx
 
+### Changed
+- Calling `json_encode()` on a `PackedBox` or `PackedItem` now additionally serialises the entire underlying
+  `Box`/`Item` where those objects also implement `JsonSerializable`. Previously the serialisation only included the
+  key values from the `Box`/`Item` interfaces themselves.
+
 ## [3.10.0] - 2022-09-10
 ### Added
 - Added `ItemSorter`, `BoxSorter` and `PackedBoxSorter` to allow calling applications to have better control over
diff --git a/src/PackedBox.php b/src/PackedBox.php
index edf1d7be..03f53363 100644
--- a/src/PackedBox.php
+++ b/src/PackedBox.php
@@ -14,6 +14,8 @@
 use function iterator_to_array;
 use function max;
 use function round;
+use function array_merge;
+use function is_array;
 
 /**
  * A "box" with items.
@@ -216,13 +218,27 @@ public function __construct(Box $box, PackedItemList $packedItemList)
     #[ReturnTypeWillChange]
     public function jsonSerialize()/* : mixed */
     {
+        $userValues = [];
+
+        if ($this->box instanceof JsonSerializable) {
+            $userSerialisation = $this->box->jsonSerialize();
+            if (is_array($userSerialisation)) {
+                $userValues = $userSerialisation;
+            } else {
+                $userValues = ['extra' => $userSerialisation];
+            }
+        }
+
         return [
-            'box' => [
-                'reference' => $this->box->getReference(),
-                'innerWidth' => $this->box->getInnerWidth(),
-                'innerLength' => $this->box->getInnerLength(),
-                'innerDepth' => $this->box->getInnerDepth(),
-            ],
+            'box' => array_merge(
+                $userValues,
+                [
+                    'reference' => $this->box->getReference(),
+                    'innerWidth' => $this->box->getInnerWidth(),
+                    'innerLength' => $this->box->getInnerLength(),
+                    'innerDepth' => $this->box->getInnerDepth(),
+                ]
+            ),
             'items' => iterator_to_array($this->items),
         ];
     }
diff --git a/src/PackedItem.php b/src/PackedItem.php
index 3adde26d..b915145b 100644
--- a/src/PackedItem.php
+++ b/src/PackedItem.php
@@ -11,6 +11,9 @@
 use JsonSerializable;
 use ReturnTypeWillChange;
 
+use function array_merge;
+use function is_array;
+
 /**
  * A packed item.
  *
@@ -131,6 +134,17 @@ public function toOrientatedItem(): OrientatedItem
     #[ReturnTypeWillChange]
     public function jsonSerialize()/* : mixed */
     {
+        $userValues = [];
+
+        if ($this->item instanceof JsonSerializable) {
+            $userSerialisation = $this->item->jsonSerialize();
+            if (is_array($userSerialisation)) {
+                $userValues = $userSerialisation;
+            } else {
+                $userValues = ['extra' => $userSerialisation];
+            }
+        }
+
         return [
             'x' => $this->x,
             'y' => $this->y,
@@ -138,13 +152,16 @@ public function jsonSerialize()/* : mixed */
             'width' => $this->width,
             'length' => $this->length,
             'depth' => $this->depth,
-            'item' => [
-                'description' => $this->item->getDescription(),
-                'width' => $this->item->getWidth(),
-                'length' => $this->item->getLength(),
-                'depth' => $this->item->getDepth(),
-                'keepFlat' => $this->item->getKeepFlat(),
-            ],
+            'item' => array_merge(
+                $userValues,
+                [
+                    'description' => $this->item->getDescription(),
+                    'width' => $this->item->getWidth(),
+                    'length' => $this->item->getLength(),
+                    'depth' => $this->item->getDepth(),
+                    'keepFlat' => $this->item->getKeepFlat(),
+                ]
+            ),
         ];
     }
 }
diff --git a/tests/PackedBoxListTest.php b/tests/PackedBoxListTest.php
index d228203b..3e43cfd2 100644
--- a/tests/PackedBoxListTest.php
+++ b/tests/PackedBoxListTest.php
@@ -190,6 +190,6 @@ public function testJsonSerialize(): void
         $packedBoxList = new PackedBoxList();
         $packedBoxList->insert($packedBox);
 
-        self::assertJsonStringEqualsJsonString('[{"box":{"reference":"Box","innerWidth":10,"innerLength":10,"innerDepth":20},"items":[{"x":0,"y":0,"z":0,"width":4,"length":10,"depth":10,"item":{"description":"Item","width":4,"length":10,"depth":10,"keepFlat":true}}]}]', json_encode($packedBoxList));
+        self::assertJsonStringEqualsJsonString('[{"box":{"reference":"Box","innerWidth":10,"innerLength":10,"innerDepth":20,"maxWeight":10,"emptyWeight":10},"items":[{"x":0,"y":0,"z":0,"width":4,"length":10,"depth":10,"item":{"description":"Item","width":4,"length":10,"depth":10,"keepFlat":true,"weight":10}}]}]', json_encode($packedBoxList));
     }
 }
diff --git a/tests/PackedBoxTest.php b/tests/PackedBoxTest.php
index 2e4a7c21..5278bf7d 100644
--- a/tests/PackedBoxTest.php
+++ b/tests/PackedBoxTest.php
@@ -98,6 +98,6 @@ public function testJsonSerialize(): void
 
         $packedBox = new PackedBox($box, $boxItems);
 
-        self::assertJsonStringEqualsJsonString('{"box":{"reference":"Box","innerWidth":10,"innerLength":10,"innerDepth":20},"items":[{"x":0,"y":0,"z":0,"width":4,"length":10,"depth":10,"item":{"description":"Item","width":4,"length":10,"depth":10,"keepFlat":true}}]}', json_encode($packedBox));
+        self::assertJsonStringEqualsJsonString('{"box":{"reference":"Box","innerWidth":10,"innerLength":10,"innerDepth":20,"maxWeight":10,"emptyWeight":10},"items":[{"x":0,"y":0,"z":0,"width":4,"length":10,"depth":10,"item":{"description":"Item","width":4,"length":10,"depth":10,"keepFlat":true,"weight":10}}]}', json_encode($packedBox));
     }
 }
diff --git a/tests/PackedItemTest.php b/tests/PackedItemTest.php
index a7fba622..e581cc77 100644
--- a/tests/PackedItemTest.php
+++ b/tests/PackedItemTest.php
@@ -33,6 +33,6 @@ public function testVolumeCalculation(): void
     public function testJsonSerialize(): void
     {
         $packedItem = new PackedItem(new TestItem('Item', 1, 2, 3, 10, false), 100, 20, 300, 3, 5, 7);
-        self::assertJsonStringEqualsJsonString('{"x":100,"y":20,"z":300,"width":3,"length":5,"depth":7,"item":{"description":"Item","width":1,"length":2,"depth":3,"keepFlat":false}}', json_encode($packedItem));
+        self::assertJsonStringEqualsJsonString('{"x":100,"y":20,"z":300,"width":3,"length":5,"depth":7,"item":{"description":"Item","width":1,"length":2,"depth":3,"keepFlat":false, "weight":10}}', json_encode($packedItem));
     }
 }