The invention relates to a method for measuring the quantity of chemical species contained in a high-temperature gas, and especially the quantity of CO and/or CO.sub.2 contained in a gas emitted from a metal treating furnace, especially a light-arc furnace (EAF) or a converter (BOF). According to the invention, part of the gas to be analysed is extracted, the temperature of said gas is reduced to at least 300.degree. C., preferably to a temperature that is lower than or equal to 200.degree. C., in such a way as to obtain a at a temperature between 300.degree. C., preferably 200.degree. C., and the ambient temperature, and at least the quantity of CO and/or CO.sub.2 said gas is then measured by means of the coherent light signal which is emitted by a laser diode through the gas and is retrieved on emergence from the gas.