diff --git a/examples/mp4dump.rs b/examples/mp4dump.rs
index 6a97d9a0..657e5478 100644
--- a/examples/mp4dump.rs
+++ b/examples/mp4dump.rs
@@ -56,7 +56,9 @@ fn get_boxes(file: File) -> Result<Vec<Box>> {
         if let Some(mehd) = &mvex.mehd {
             boxes.push(build_box(mehd));
         }
-        boxes.push(build_box(&mvex.trex));
+        for trex in mvex.trexs.iter() {
+            boxes.push(build_box(trex));
+        }
     }
 
     // trak.
diff --git a/src/mp4box/moov.rs b/src/mp4box/moov.rs
index ac19381a..80f9953f 100644
--- a/src/mp4box/moov.rs
+++ b/src/mp4box/moov.rs
@@ -38,6 +38,9 @@ impl MoovBox {
         if let Some(udta) = &self.udta {
             size += udta.box_size();
         }
+        if let Some(mvex) = &self.mvex {
+            size += mvex.box_size();
+        }
         size
     }
 }
@@ -134,6 +137,9 @@ impl<W: Write> WriteBox<&mut W> for MoovBox {
         for trak in self.traks.iter() {
             trak.write_box(writer)?;
         }
+        if let Some(mvex) = &self.mvex {
+            mvex.write_box(writer)?;
+        }
         if let Some(meta) = &self.meta {
             meta.write_box(writer)?;
         }
diff --git a/src/mp4box/mvex.rs b/src/mp4box/mvex.rs
index 8be683b8..58d33329 100644
--- a/src/mp4box/mvex.rs
+++ b/src/mp4box/mvex.rs
@@ -7,16 +7,26 @@ use crate::mp4box::{mehd::MehdBox, trex::TrexBox};
 #[derive(Debug, Clone, PartialEq, Eq, Default, Serialize)]
 pub struct MvexBox {
     pub mehd: Option<MehdBox>,
-    pub trex: TrexBox,
+
+    #[serde(rename = "trex")]
+    pub trexs: Vec<TrexBox>,
 }
 
 impl MvexBox {
     pub fn get_type(&self) -> BoxType {
-        BoxType::MdiaBox
+        BoxType::MvexBox
     }
 
     pub fn get_size(&self) -> u64 {
-        HEADER_SIZE + self.mehd.as_ref().map(|x| x.box_size()).unwrap_or(0) + self.trex.box_size()
+        let mut size = HEADER_SIZE;
+
+        size += self.mehd.as_ref().map_or(0, |x| x.box_size());
+
+        for trex in self.trexs.iter() {
+            size += trex.box_size();
+        }
+
+        size
     }
 }
 
@@ -44,7 +54,7 @@ impl<R: Read + Seek> ReadBox<&mut R> for MvexBox {
         let start = box_start(reader)?;
 
         let mut mehd = None;
-        let mut trex = None;
+        let mut trexs = Vec::new();
 
         let mut current = reader.stream_position()?;
         let end = start + size;
@@ -63,7 +73,7 @@ impl<R: Read + Seek> ReadBox<&mut R> for MvexBox {
                     mehd = Some(MehdBox::read_box(reader, s)?);
                 }
                 BoxType::TrexBox => {
-                    trex = Some(TrexBox::read_box(reader, s)?);
+                    trexs.push(TrexBox::read_box(reader, s)?);
                 }
                 _ => {
                     // XXX warn!()
@@ -74,16 +84,13 @@ impl<R: Read + Seek> ReadBox<&mut R> for MvexBox {
             current = reader.stream_position()?;
         }
 
-        if trex.is_none() {
+        if trexs.is_empty() {
             return Err(Error::BoxNotFound(BoxType::TrexBox));
         }
 
         skip_bytes_to(reader, start + size)?;
 
-        Ok(MvexBox {
-            mehd,
-            trex: trex.unwrap(),
-        })
+        Ok(MvexBox { mehd, trexs })
     }
 }
 
@@ -95,7 +102,10 @@ impl<W: Write> WriteBox<&mut W> for MvexBox {
         if let Some(mehd) = &self.mehd {
             mehd.write_box(writer)?;
         }
-        self.trex.write_box(writer)?;
+
+        for trex in self.trexs.iter() {
+            trex.write_box(writer)?;
+        }
 
         Ok(size)
     }
diff --git a/src/reader.rs b/src/reader.rs
index 5a39eebb..567cb2f2 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -94,16 +94,24 @@ impl<R: Read + Seek> Mp4Reader<R> {
 
         // Update tracks if any fragmented (moof) boxes are found.
         if !moofs.is_empty() {
-            let mut default_sample_duration = 0;
-            if let Some(ref moov) = moov {
-                if let Some(ref mvex) = &moov.mvex {
-                    default_sample_duration = mvex.trex.default_sample_duration
-                }
-            }
+            // Grab the mvex box if it exists.
+            let mvex = moov.as_ref().and_then(|moov| moov.mvex.as_ref());
 
             for moof in moofs.iter() {
                 for traf in moof.trafs.iter() {
                     let track_id = traf.tfhd.track_id;
+
+                    // Get the default sample duration for the indicated track.
+                    // This is buried in the optional mvex box, in a trex box per track.
+                    let default_sample_duration = mvex
+                        .and_then(|mvex| {
+                            mvex.trexs
+                                .iter()
+                                .find(|trex| trex.track_id == track_id)
+                                .map(|trex| trex.default_sample_duration)
+                        })
+                        .unwrap_or(0);
+
                     if let Some(track) = tracks.get_mut(&track_id) {
                         track.default_sample_duration = default_sample_duration;
                         track.trafs.push(traf.clone())
diff --git a/src/types.rs b/src/types.rs
index 19fd40b3..a26c0b5a 100644
--- a/src/types.rs
+++ b/src/types.rs
@@ -657,20 +657,15 @@ pub fn creation_time(creation_time: u64) -> u64 {
     }
 }
 
-#[derive(Debug, Clone, PartialEq, Eq, Serialize)]
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Default)]
 pub enum DataType {
+    #[default]
     Binary = 0x000000,
     Text = 0x000001,
     Image = 0x00000D,
     TempoCpil = 0x000015,
 }
 
-impl std::default::Default for DataType {
-    fn default() -> Self {
-        DataType::Binary
-    }
-}
-
 impl TryFrom<u32> for DataType {
     type Error = Error;
     fn try_from(value: u32) -> Result<DataType> {