Pt (0.1 wt.%) on Al2O3, TiO2, and MgO supports were used for the low temperature combustion of methanol in both the absence and the presence of ammonia. Without NH3 in the gas the performance for the complete combustion decreases in the order Pt/MgO greater than or equal to Pt/TiO2 much greater than Pt/Al2O3 comparing a fixed amount of Pt surface area. In the presence of ammonia strong deactivation of Pt/Al2O3 and Pt/TiO2 is observed, whilst almost no deactivation of Pt/MgO occurs. The results are explained considering CO chemisorption and XPS measurements as well as the TPD profiles generated by adsorbed ammonia and methanol. A comparison of the TPD profiles of the N-2 which is formed from adsorbed ammonia on supported Pt and the support, respectively, reveals that Pt on Al2O3 and TiO2 interacts with the support surface, possibly through Pt-O-support bonding. The inference is supported by the XPS measurements showing that the Pt on Al2O3 and TiO2, as opposed to that on MgO, is slightly positively charged, making the interaction between ammonia and Pt stronger. Additional insight into the deactivation cause is obtained from TPD experiments with adsorbed methanol. These experiments reveal that on both Pt/Al2O3 and Pt/TiO2 there is competition between methanol and ammonia for adsorption sites, which is not the case on Pt/MgO.