Decomposition and oxidation of methanol on Pt(111) have been examined between 300 and 650 K in the millibar pressure range using in situ ambient-pressure X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction spectroscopy (TPRS). It was found that even in the presence of oxygen, the methanol decomposition on platinum proceeds through two competitive routes: fast dehydrogenation to CO and slow decomposition via the C–O bond scission. The rate of the second route is significant in the millibar pressure range, which leads to a blocking of the platinum surface by carbon and to the prevention of further methanol conversion. As a result, without oxygen, the activity of Pt(111) converted to a turnover frequency is 0.3 s–1 at 650 K. The activity strongly increases with oxygen content, achieving 20 s–1 in an oxygen-rich mixture. The main products of methanol oxidation were CO, CO2, H2, and H2O. The CO selectivity as well as the H2 selectivity decrease with the increase in oxygen content. It means that the main reaction route is the methanol dehydrogenation to CO and hydrogen; however, in the presence of oxygen, CO oxidizes to CO2 and hydrogen oxidizes to water. At room temperature, the C1s spectra contain weak features of formate species. This finding points out that the “non-CO-involved” pathway of methanol oxidation realizes on platinum as well. However, the TPRS data indicate that at least under the oxygen-deficient conditions the methanol dehydrogenation pathway dominates. A detailed reaction mechanism of the decomposition and oxidation of methanol agreeing with XPS and TPRS data is discussed.