According to a number of international and European regulations a gas, or mixture of gases, is classified as an oxidizing substance if it is more oxidizing than air. For classification it is necessary to have a method to determine reliable and comparable numerical data for the oxidizing ability of gases and a reliable calculation method for mixtures of gases. In ISO 10156 “Gases and gas mixtures — Determination of fire potential and oxidizing ability for the selection of cylinder valve outlets” a test method is given using pieces of plastic as combustible reference material. This method was tested in the past by Hoechst AG, Germany, and BAM and failed in the case of oxidizing gases other than air and oxygen. Therefore BAM developed a new test method in cooperation with ISO subcommittee ISO/TC58/SC2. The new procedure is based on the same apparatus and criteria applied for determination of flammability in ISO 10156. The reference combustible is the gas “Ethane”. This paper describes the new test procedure and gives “coefficients of oxygen equivalency (Ci)” for the oxidizing gases air, nitrous oxide, nitric oxide, chlorine, fluorine and nitrogen trifluoride. In the case of nitrous oxide, oxygen, air and nitrogen, a calculation method for gas mixtures was checked.