Plasmonic nanosystems can enhance photochemical processes taking place in their neighborhood on account of several plasmonically mediated processes, among them: (i) the enhancement of the optical fields in the vicinity of the nanosystem and (ii) the intervention of energetic electrons and/or holes in the redox chemistry of nearby molecules pursuant to plasmon excitation and decay. We describe a series of experiments in which tetrachloroplatinate anions are reduced to zerovalent Pt at the surface of gold nanoparticles and nanoparticle assemblies. We demonstrate that the spatial pattern of deposited Pt cannot be understood in terms of enhanced fields alone but likely involves hot electrons whose momenta are initially aligned with the electric vector of the polarized light used to excite plasmons but become randomized on account of electron–electron interactions and scattering within the metallic nanoparticles before the electrons’ energy is substantially thermalized. These conclusions are supported by numer...