The effects of gold nanoparticles deposited on titanium dioxide on the photocatalytic oxidative degradation of two organic substrates, i.e. formic acid and the azo dye Acid Red 1, and on the parallel O₂ reduction yielding hydrogen peroxide have been investigated under visible light irradiation. The method employed to reduce Au(III) to metallic gold in the preparation of Au/TiO₂ photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO₂. The presence of gold on TiO₂ facilitates both the electron transfer to O₂ and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO₂˙⁻ intermediate species may contribute in maintaining gold in metallic form. The controversial results obtained in the photocatalytic degradation of the dye were rationalised by taking into account that with this substrate, which mainly undergoes oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidise gold nanoparticles, which consequently act as recombination centres of photoproduced charge carriers.