The mechanism of CO oxidation on deposited Pt is analyzed by electromotive forces (emf) measurement for a solid electrolyte concentration cell where the partial pressures, PO 2 and PCO, are between 5 Pa and 50 kPa, and the temperature range is 520 to 680 °K. The emf is expressed as a function of the surface oxygen and CO concentrations adsorbed on Pt, with reasonable assumptions. Thus, emf dependence on PO 2 and PCO shows surface species concentrations during reaction as functions of PO 2 and PCO. As a result, in the region where CO is scarce compared to oxygen, surface CO concentration is determined to be proportional to PO 2 γ PCO δ , under the assumption that the surface oxygen concentration is constant. Here, δ is found to increase from 1 to about 1.6 with PCO, γ is −1.4 at lower temperatures (about 580 °K), and −1 at higher temperatures (about 670 °K). This dependence is considered in terms of residence time and surface mobility for CO on Pt. In the region where CO is sufficient, the surface CO concentration is almost saturated and that of surface oxygen is expressed as k P O 2 1.2 P CO 1.1 . Thus, CO oxidation in that case is considered to proceed with the rate-determining step of surface reaction between adsorbed CO and associatively adsorbed oxygen. The transient emf observed as PO 2 or PCO increased or decreased stepwise also reveals that CO oxidation occurs via a Langmuir-Hinshelwood mechanism.