We compare catalytic methanol oxidation reactions in the gas and liquid phases by focusing on the kinetic effects of platinum nanoparticle size and metal/support interactions. Under the reaction conditions at 60 A degrees C, methanol can be oxidized to multiple products including carbon dioxide (full-oxidation product), formaldehyde (partial-oxidation product) and methyl formate (partial-oxidation and coupling product). We use 2, 4, 6 and 8 nm platinum nanoparticles supported on mesoporous silica as catalysts to study the size effect, and 2.5 nm platinum nanoparticles supported on mesoporous SiO2, Co3O4, MnO2, Fe2O3, NiO and CeO2 to study the metal/oxide interface effect. We find that all three products are formed with comparable selectivities in the gas phase, but in the liquid phase formaldehyde is the dominant product. While the influence of size on activity is not substantial in the gas phase, the liquid-phase reaction rates monotonically increase by a factor of 6 in the size range of 2-8 nm. The reaction rates in the gas phase are dramatically affected by the strong interactions between the platinum nanoparticles and transition metal oxide supports. While the Pt/MnO2 is 135 times less active, the Pt/CeO2 is 12 times more active, both compared to the Pt/SiO2. However in the liquid phase, the support effect is less significant, with the most active catalyst Pt/MnO2 exhibiting an enhancement factor of 2.5 compared to the Pt/SiO2. Our results suggest that the kinetic effects of platinum nanoparticle size and metal/support interactions can be totally different between the solid/gas and solid/liquid interfaces even for the same chemical reaction.