Au nanoparticles prepared by deposition–precipitation were evaluated in promoting photocatalytic activity of TiO2 (P25) in the oxidation of methylcyclohexane. At 375 nm and in particular at 425 nm, Au was found to significantly enhance the rate induced by P25. Illumination of Au-promoted P25 at 525 nm did not result in any measureable activity. To validate whether the enhancement at 425 nm is purely catalytic, or if plasmonic effects are relevant, we compared the rates of Au/TiO2 with Pt-promoted TiO2 at 375 and 425 nm. At 375 nm, Pt nanoparticles induced larger catalytic effects than Au nanoparticles, whereas the rate enhancement induced by Pt was much lower than of Au at 425 nm. We assign the thus demonstrated Au based plasmonic phenomena at 425 nm to the so-called plasmon resonance energy transfer, rather than to hot electron transfer, given the absence of activity at 525 nm.