The international chemical weapons watchdog says that all the precursor chemicals for sarin gas removed from Syria have been destroyed.
The 580 tonnes of chemicals were neutralised on board a specially built American ship in the Mediterranean Sea.
The Organisation for the Prohibition of Chemical Weapons (OPCW) said the ship's crew have now begun to neutralise 22 tonnes of sulphur mustard.
The process is part of a Russian-US deal to eliminate Syria's arsenal.
The deal was drawn up after hundreds of people died in a sarin attack in the Ghouta area outside Damascus on 21 August last year.
The US ship, the MV Cape Ray, was fitted out with two $5m (£3m) mobile hydrolysis systems, developed in 2013 by US military researchers, specifically to deal with the chemicals.
OPCW chief ambassador Ahmet Uzumcu announced the completion of this stage of the process on Wednesday.
The two hydrolysis systems neutralised the precursor chemicals - methylphosphonyl difluoride - by mixing them with fresh water and "reagents" and then heating the mixture to reduce its toxicity.
Once the ship has processed the remaining sulphur mustard chemicals, all of the resulting effluents will be transported to Finland and Germany for disposal at land-based facilities.
As the process to deal with the chemicals is nearing its end, the United Nations has already raised concerns over the use of chlorine as a weapon by President Bashar al-Assad's forces.
Chlorine was not included in the list of toxic agents declared by Syria as part of its agreement to hand over its chemical stockpile.
Although it has many legitimate industrial functions, the use of chlorine as a weapon is banned by the Chemical Weapons Convention.
If high concentrations of the chemical enter the lungs it can cause death.
President Assad's government has denied using chlorine but an OCPW investigation found evidence that it was being used in "a systematic manner".