The heterogeneous reaction CO+O→CO2 on water ice and various inorganic oxide (SiO2, Al2O3, Fe2O3, Fe3O4, CaO, MgO, and TiO2) surfaces has been investigated in order to evaluate the proposition that this heterogeneous reaction may contribute to the CO2 stability in the Martian atmosphere. The reaction probabilities were measured at both 196 K and 294 K by using a fast flow-tube reactor coupled to an electron-impact ionization mass spectrometer. The atomic oxygen was generated by passing a mixture of O2 and He through a microwave discharge at a side-arm inlet and then allowed to react with CO over these substrates in the reactor. The reaction product, CO2, was monitored as a function of time at the downstream end of the flow-tube reactor. The measured reaction probabilities were determined from the growth rates of CO2 signal and were found to range from 1.6¿10-7 to 1.0¿10-5 on the basis of the geometric area of the reactor. If we adopt a typical reaction probability of 5¿10-7 for the heterogeneous CO+O reaction on Martian aerosols at 196 K, a simple calculation suggests that this mechanism plays a negligible role in CO2 recycling as compared to that of the homogeneous CO+OH reaction. ¿ 1997 American Geophysical Union