Samples from 10 victims were analysed at four laboratories, OPCW head Ahmet Uzumcu said.
The attack on the rebel-held town killed at least 87 people.
The Syrian military denied using any chemical agents.
Its ally Russia, meanwhile, said an air strike hit a rebel depot full of chemical munitions - but this suggestion has been widely rejected.
The US responded to the attack by launching air strikes on a Syrian military airfield.
Footage following the incident showed civilians, many of them children, choking and foaming at the mouth.
