Representatives from the OPCW say samples have been removed for analysis.
"The OPCW will evaluate the situation and consider future steps including another possible visit to Douma", the organization said in a press release.
"According to information we have, the special OPCW mission arrived on the morning of 21 April in the city of Douma at the sites suspected of having toxic substances", the Russia's foreign ministry said in a statement.
In 2013, however, following accusations of launching chemical attacks, which reportedly also took place in the Damascus suburbs, Syria relinquished its sizable arsenal of chemical weapons, which was subsequently destroyed under OPCW auspices - an act which later led to the OPCW being awarded the Nobel Peace Prize.
The OPCW mission aims to establish whether chemical weapons were used, but is not mandated to apportion blame for the attack.
Worldwide investigators have finally arrived at the site of a suspected chemical attack in Syria.
Last week Western states bombed several Syrian government sites in retaliation for the suspected attack on 7 April.
A United Nations security team touring Douma on Tuesday came under small arms and explosives fire, leading the OPCW to postpone its visit.
Eyewitness reports, photos, and videos of limp children and people struggling to breathe make it apparent that an atrocity occurred in Douma (though Syria and its allies claim it's all a hoax).
Images that emerged from Douma in the hours after the attack showed lifeless bodies collapsed in crowded rooms, some with foam around their noses and mouths.
But tragically, Syrians have been subjected to horrors throughout their country's seven-year civil war, and in the world of worldwide relations, the nature of the chemical weapons allegedly used by Bashar al-Assad's forces last weekend matters. North Syria is divided between opposition, Turkish and al-Qaeda control.
The station said there could be 3,200 rebels leaving the three towns on Saturday.
Tens of thousands of people have been transferred out of the areas around Damascus in recent weeks as the government reconsolidates control. It said the evacuations will continue for three days.
But they spent a week in the capital, unable to access Douma just a few miles to the east, even as groups of journalists visited the town on government tours.