Fine metal nanoparticles (2-3 nm; Au, Pt, and alloyed Au-Pt) with a narrow size distribution were deposited on active TiO₂ through a facile chemical reduction method. Compared to the bare TiO₂, a remarkable enhancement of up to 10-fold for photocatalytic hydrogen evolution was achieved on the alloyed nanocomposites. By using core level and valence band XPS analysis, two electronic properties are shown to contribute to the promoted photocatalytic activity: stronger metal-support interaction between the alloyed structures and TiO₂ and higher electron population on the Au-Pt/TiO₂ photocatalysts in comparison with the bare TiO₂. Moreover, an improved charge separation over TiO₂ using Au-Pt nanoparticles was clearly evidenced by the significant increase of photocurrent responses obtained from the photoelectrochemical measurements. For the first time, in situ ¹³C and ¹H NMR spectroscopy was applied to monitor the gas-liquid-solid photocatalytic reactions under real working conditions. Via a two-electron oxidation pathway, the surface-adsorbed methanol was first oxidized to formaldehyde, followed by spontaneous hydrolysis and methanolysis to methanediol and methoxymethanol, rather than methyl formate and formic acid that have been previously reported in gaseous CH₃OH photocatalysis. The in situ monitoring also revealed that deposition of metal NPs would not alter the reaction pathways while making the reaction faster compared to the bare TiO₂. ; 8 page(s)