diff --git a/src/api/s3_put.rs b/src/api/s3_put.rs
index 1939f2cf..7aec12ed 100644
--- a/src/api/s3_put.rs
+++ b/src/api/s3_put.rs
@@ -100,7 +100,7 @@ pub async fn handle_put(
 	// Transfer data and verify checksum
 	let tx_result = read_and_put_blocks(
 		&garage,
-		version,
+		&version,
 		1,
 		first_block,
 		first_block_hash,
@@ -174,7 +174,7 @@ fn ensure_checksum_matches(
 
 async fn read_and_put_blocks(
 	garage: &Arc<Garage>,
-	version: Version,
+	version: &Version,
 	part_number: u64,
 	first_block: Vec<u8>,
 	first_block_hash: Hash,
@@ -392,7 +392,7 @@ pub async fn handle_put_part(
 	let first_block_hash = blake2sum(&first_block[..]);
 	let (_, data_md5sum, data_sha256sum) = read_and_put_blocks(
 		&garage,
-		version,
+		&version,
 		part_number,
 		first_block,
 		first_block_hash,
@@ -400,6 +400,7 @@ pub async fn handle_put_part(
 	)
 	.await?;
 
+	// Verify that checksums map
 	ensure_checksum_matches(
 		data_md5sum.as_slice(),
 		data_sha256sum,
@@ -407,8 +408,14 @@ pub async fn handle_put_part(
 		content_sha256,
 	)?;
 
+	// Store part etag in version
+	let data_md5sum_hex = hex::encode(data_md5sum);
+	let mut version = version;
+	version.parts_etags.put(part_number, data_md5sum_hex.clone());
+	garage.version_table.insert(&version).await?;
+
 	let response = Response::builder()
-		.header("ETag", format!("\"{}\"", hex::encode(data_md5sum)))
+		.header("ETag", format!("\"{}\"", data_md5sum_hex))
 		.body(Body::from(vec![]))
 		.unwrap();
 	Ok(response)
@@ -463,43 +470,46 @@ pub async fn handle_complete_multipart_upload(
 	};
 
 	// Check that the list of parts they gave us corresponds to the parts we have here
-	// TODO: check MD5 sum of all uploaded parts? but that would mean we have to store them somewhere...
-	let mut parts = version
-		.blocks
+	println!("Expected parts from request: {:?}", body_list_of_parts);
+	println!("Parts stored in version: {:?}", version.parts_etags.items());
+	let parts = version
+		.parts_etags
 		.items()
 		.iter()
-		.map(|x| x.0.part_number)
-		.collect::<Vec<_>>();
-	parts.dedup();
+		.map(|pair| (&pair.0, &pair.1));
 	let same_parts = body_list_of_parts
 		.iter()
-		.map(|x| &x.part_number)
-		.eq(parts.iter());
+		.map(|x| (&x.part_number, &x.etag))
+		.eq(parts);
 	if !same_parts {
 		return Err(Error::BadRequest(format!("We don't have the same parts")));
 	}
 
-	// ETag calculation: we produce ETags that have the same form as
-	// those of S3 multipart uploads, but we don't use their actual
-	// calculation for the first part (we use random bytes). This
-	// shouldn't impact compatibility as the S3 docs specify that
-	// the ETag is an opaque value in case of a multipart upload.
-	// See also: https://teppen.io/2018/06/23/aws_s3_etags/
+	// Calculate etag of final object
+	// To understand how etags are calculated, read more here:
+	// https://teppen.io/2018/06/23/aws_s3_etags/
 	let num_parts = version.blocks.items().last().unwrap().0.part_number
 		- version.blocks.items().first().unwrap().0.part_number
 		+ 1;
+	let mut etag_md5_hasher = Md5::new();
+	for (_, etag) in version.parts_etags.items().iter() {
+		etag_md5_hasher.update(etag.as_bytes());
+	}
 	let etag = format!(
 		"{}-{}",
-		hex::encode(&rand::random::<[u8; 16]>()[..]),
+		hex::encode(etag_md5_hasher.finalize()),
 		num_parts
 	);
 
+	// Calculate total size of final object
 	let total_size = version
 		.blocks
 		.items()
 		.iter()
 		.map(|x| x.1.size)
 		.fold(0, |x, y| x + y);
+
+	// Write final object version
 	object_version.state = ObjectVersionState::Complete(ObjectVersionData::FirstBlock(
 		ObjectVersionMeta {
 			headers,
@@ -512,6 +522,7 @@ pub async fn handle_complete_multipart_upload(
 	let final_object = Object::new(bucket.clone(), key.clone(), vec![object_version]);
 	garage.object_table.insert(&final_object).await?;
 
+	// Send response saying ok we're done
 	let mut xml = String::new();
 	writeln!(&mut xml, r#"<?xml version="1.0" encoding="UTF-8"?>"#).unwrap();
 	writeln!(
diff --git a/src/model/version_table.rs b/src/model/version_table.rs
index 26abb64e..7ccc6a33 100644
--- a/src/model/version_table.rs
+++ b/src/model/version_table.rs
@@ -16,8 +16,11 @@ pub struct Version {
 	pub uuid: UUID,
 
 	// Actual data: the blocks for this version
+	// In the case of a multipart upload, also store the etags
+	// of individual parts and check them when doing CompleteMultipartUpload
 	pub deleted: crdt::Bool,
 	pub blocks: crdt::Map<VersionBlockKey, VersionBlock>,
+	pub parts_etags: crdt::Map<u64, String>,
 
 	// Back link to bucket+key so that we can figure if
 	// this was deleted later on
@@ -31,6 +34,7 @@ impl Version {
 			uuid,
 			deleted: deleted.into(),
 			blocks: crdt::Map::new(),
+			parts_etags: crdt::Map::new(),
 			bucket,
 			key,
 		}
@@ -82,8 +86,10 @@ impl CRDT for Version {
 
 		if self.deleted.get() {
 			self.blocks.clear();
+			self.parts_etags.clear();
 		} else {
 			self.blocks.merge(&other.blocks);
+			self.parts_etags.merge(&other.parts_etags);
 		}
 	}
 }