Various driving conditions determine major changes in IC-engined vehicles polluting emissions. The generalized use of three-way catalysts on gasoline-engined vehicles and, in the near future, oxidation catalysts on diesel-engined vehicles, is likely to enable the control of such changes. Thus, the influence of driving conditions on the efficiency of such devices was studied in a research program proposed by RENAULT and carried out in collaboration with LHVP and UTAC. This study was performed using a chassis dynamometer with the measurement of both regulated (CO, HC, NOx) and some non-regulated pollutants (sulphur oxides, aldehydes, monocyclic aromatic hydrocarbons, polycyclic aromatic hydrocarbons, nitrated or otherwise). Simultaneously, the mutagenous properties of such emissions were assessed using the Ames test. Test vehicles were operated under urban cold or hot start conditions (a sub-unit of the European cycle) and under extra-urban hot start conditions( another sub-unit of the European cycle). For the two engine types, the extra-urban cycle raises catalyst efficiency to a maximum: >95% for gasoline cars and ∼ 90% for organic compounds of diesel cars. Conversely, for cold start in the urban cycle, the catalyst-related decrease in pollution levels is less significant. It is of the order of 85% for gasoline-engined vehicles and <40% for diesel-engined vehicles. Finally, for the hot urban cycle, the performances of the catalysts installed on gasoline-engined vehicles are similar to those obtained for the extra-urban cycle. But with diesel-engined vehicles, the efficiency of such a device is close to that obtained for cold conditions. Catalytic post-combustion seems to be an efficient system for reducing the mutageneous effect of emissions, in particular on diesel-engined vehicles. Les diverses conditions de conduite auxquelles sont soumis les véhicules à moteur thermique déterminent de façon majeure les variations des émissions polluantes. Aussi, l'influence des conditions de conduite sur l'efficacité des dispositifs de dépollution a été étudie au cours dun programme de recherche développé par RENAULT et exécuté en collaboration avec le LHVP et l'UTAC. Cette étude a été réalisée avec des véhicules RENAULT fonctionnant sur banc à rouleaux en mesurant les polluants réglementés et certains polluants non-réglementés. Parallélement, les propriétés mutagènes des émissions ont été appréciées par un test biologique (test d'Ames). Les véhicules d'essais ont fonctionné en cycle urbain moteur froid ou chaud au démarrage et en cycle extra-urbain moteur chaud au démarrage.