The interim report by the Organization for the Prohibition of Chemical Weapons (OPCW) said “various chlorinated organic chemicals” had been found but there was no evidence of nerve agents.
Dozens of civilians were killed in the attack on the rebel-held town in the Eastern Ghouta region near Damascus.
The Syrian government denies carrying out any chemical weapons attacks.
Following the Douma attack, US, British and French warplanes launched strikes against government military targets.
The OPCW was recently given new powers by member states to name those responsible for using chemical weapons. Its investigation into the Douma attack is continuing.
Medics in Douma reported on 7 April that more than 500 patients had been brought to medical facilities with symptoms suggesting exposure to a chemical agent.
Rescue workers also reported a strong smell of chlorine in the air following an air strike that targeted two locations in Douma.
The OPCW sent a fact-finding mission to Douma about a week later.
“Along with explosive residues, various chlorinated organic chemicals were found in samples from two sites,” the preliminary report said.
“Work by the team to establish the significance of these results is on-going. The FFM (fact-finding mission) team will continue its work to draw final conclusions.”
The report said two samples from gas cylinders recovered at the scene tested positive for chlorine.
Following the attack, a UN investigation said evidence pointed to the use of chlorine but that some people had exhibited symptoms more consistent with exposure to a nerve agent.
The OPCW report, however, said that “no organophosphorous nerve agents or their degradation products were detected” at the sites.
The five-year siege of the Eastern Ghouta, an agricultural region outside the capital where at least 265,000 civilians lived, was the longest in modern history.
It came to an end in April, after a two-month offensive by pro-government forces, backed by Russian air strikes, that decimated homes, businesses and hospitals and reportedly left more than 1,700 men, women and children dead.