The Organization for the Prohibition of Chemical Weapons (OPCW) sent a fact-finding mission to Douma in mid-April, roughly a week after the April 7 attack in the enclave near Damascus.
"Various chlorinated organic chemicals were found in samples" from two locations, the report said. It said it had found no evidence of nerve agents.
The OPCW has documented systematic use of banned munitions in Syria's civil war, including nerve agent sarin and sulphur mustard gas. It has not assigned blame for the attacks.
In Douma, OPCW inspectors visited two sites where they interviewed witnesses and took samples, which were split at their laboratory in the Netherlands and forwarded to affiliated national labs for testing.
Two samples recovered from gas cylinders at the scene tested positive for chlorine, the report said.
Washington and other Western governments blamed Syrian government forces for the attack in Douma.
The Syrian government denies using chemical weapons during the country's long civil war but a previous joint inquiry of the United Nations and the OPCW found the Syrian government used the nerve agent sarin in an April 2017 attack and has also several times used chlorine as a weapon. It blamed Islamic State militants for mustard gas use.