Gold nanoparticles supported on P25 titania (Au/TiO2) exhibit photocatalytic activity for UV and visible light (532 nm laser or polychromatic light lambda > 400 nm) water splitting. The efficiency and operating mechanism are different depending on whether excitation occurs on the titania semiconductor (gold acting as electron buffer and site for gas generation) or on the surface plasmon band of gold (photoinjection of electrons from gold onto the titania conduction band and less oxidizing electron hole potential of about -1.14 V). For the novel visible light photoactivity of Au/TiO2, it has been determined that gold loading, particle size and calcination temperature play a role in the photocatalytic activity, the most active material (Phi(H2) = 7.5% and Phi(O2) = 5.0% at 560 nm) being the catalyst containing 0.2 wt % gold with 1.87 nm average particle size and calcined at 200 degrees C.