Model nanocomposite photocatalysts consisting of undoped TiO2 films with optically active Ag or Au nanoparticles (NPs) were designed, fabricated, and examined to address the role of plasmon excitations in their performance. Different composition configurations were tested in which the NPs were either facing the reaction environment or not, and in direct contact or not with TiO2. We found, as measured for the reactions of methanol and ethylene oxidation in two different photoreactors, that composites always show enhanced activity (up to ×100 for some configurations) compared to bare TiO2. We deduced from in situ localized surface plasmon resonance spectroscopy measurements that the interfacial charge transfer from TiO2 to NPs plays a major role in the activity enhancement for composite configurations where particles are in direct contact with TiO2. Plasmonic near- and far-field effects were only observed when the plasmon resonance energy overlaps with the bandgap energy of undoped TiO2.