The catalyticreaction between CO and O 2 on a Pt (110) surface is investigated by residual gas analysis for different partial pressures of CO and O 2 and various temperatures. The surface cleanliness of the Pt crystal is monitored by Auger electron spectroscopy. The results of the reaction study are generally in agreement with previous studies for polycrystalline Pt catalysts. The temperature dependence of the formation rate of CO 2 exhibits a maximum whose position depends on the composition of the gas phase. At T=215° C two different rate laws are valid. For P O 2 >P CO the rate of CO 2 formation is proportional to P O 2 1.1 P CO −0.6 , while for P O 2 >P CO it is proportional to P O 2 0.25 P CO 0.75 . These observations support the idea that the reaction obeys an Eley-Rideal mechanism in the pressure and temperature range investigated, and the results are evaluated accordingly. The influence of segregated Si on the CO oxidationreaction is investigated and found to be small.