The oxidation of CO over thin films of a Pt-Rh alloy prepared by sequential vacuum evaporation of the metals on an inert support has been investigated at low pressures (P < 2x10(-5) mbar). The results are compared with the data on the individual surfaces of Pt and Rh. It is found that the activity of the alloy is intermediate between the activities of the individual metals. The effect of alloy formation on the catalytic activity in CO oxidation is discussed.