This document specifies a method for the determination of the gas transmission rate and gas permeability coefficient of vulcanized or thermoplastic rubber under conditions in which the overall pressure on each side of the rubber test piece is the same, using gas chromatography to determine the quantity of gas that permeates through the rubber. The method applies to vulcanized and thermoplastic rubbers of hardness not less than 35 IRHD (international rubber hardness degrees) and to both single gases and mixtures of gases.