The Organisation for the Prohibition of Chemical Weapons (OPCW) said Finland's Ekokem would destroy chemical weapon ingredients as well as some effluent left over from destruction activities aboard a US ship.
The US subsidiary of French company Veolia Environment was chosen from 14 bids from around the world to destroy the remainder of the chemicals, the OPCW said in a statement.
"The conclusion of this process represents an important step towards the full destruction of Syrian chemical weapons within the agreed timelines," it added, amid Western accusations that Damascus is dragging its feet.
Tenders to help destroy Syria's entire arsenal by a tight June 30 deadline came from a wide-range of companies, including France's Airbus, China National Chemical Corporation, Switzerland's Dottikon and US-based Paragon Waste Solutions.
Germany said in January that it would help destroy waste left over from neutralising Syria's chemicals, and Britain said in December it would destroy 150 tonnes of its industrial grade chemicals.
The OPCW says the cost of destroying chemical materials alone will run between USD 34-41 million.
Syria has declared around 700 tonnes of most dangerous chemicals, 500 tonnes of less dangerous precursor chemicals and around 122 tonnes of isopropanol, which can be used to make sarin gas.
Hydrolysis systems aboard the US ship Cape Ray are to mix the chemicals with heated water and other chemicals to break down the lethal agents, resulting in a sludge equivalent to industrial toxic waste.
