A laboratory scale analytical procedure is described to determine the efficiency of three way exhaust gas catalysts (Pt/Rh/monolith) by F.T.I.R. spectroscopy.An extruded part of monolith was used to convert synthetic gas mixtures (CO/NO/Hydrocarbons/CO2/H2O/N2) which have compositions similar to motor vehicule exhaust gases. The catalyst efficiency was determined as a function of gas temperature and equivalent air-fuel ratio (with and without perturbation).F.T.I.R. spectroscopy appears as an alternative method compared to the present one using several specific detectors. Comparative data are obtained on the conversion of CO, NO and hydrocarbons. In addition F.T.I.R. spectroscopy was capable of giving the following informations: the conversion of each hydrocarbons (in case of mixture of hydrocarbons), the transient formation of N2O and the occurence of NO2. Adapted softwares for F.T.I.R. microcomputers give conversion as a function of the temperature of the gas and equivalent air/fuel ratio.This analytical system is used to study the deactivation of three way catalysts by lead compounds.