The effect of pretreatment conditions on the oxidation state and activity of micelle-synthesized Pt nanoparticles supported on ZrO2 was studied for methanol decomposition and oxidation reactions. An O-2-pretreatment is observed to be effective for producing clean, stable, and active nanoparticles. Pt-oxide species formed during such pretreatments were found to have little influence in methanol decomposition reactions due to their tendency to reduce. However, these same species are stable during methanol oxidation and appear to take part in a Mars-van Krevelen-type of process, in which bound-oxygen (nanoparticle shell) may be replenished with oxygen from the gas phase.