Cu-porous silicon (Cu-PS) composite materials consisting in nanosized Cu clusters preferentially grown on the surface of PS were fabricated by photo-assisted deposition of Cu nanoparticles onto PS. Structural and chemical characterization of the Cu particles grown in the PS matrix has been carried out by synchrotron X-ray absorption spectroscopy, from which different reaction stages have been identified within the photo-assisted reaction. In particular, it was found that the reduction of Cu occurs in three main phases: (a) Cu nucleates homogeneously in a few seconds over the surface of PS by a coupled red-ox reaction; (b) clusters grow by new reduced ions, which tend to oxidize the previously-deposited Cu atoms making increasingly heterogeneous Cu clusters; and (c) a competitive process between nucleation of new clusters and cluster coalescence gives rise to a bulk-like Cu thin film. It was determined that the structures formed in the first two phases display surface Plasmon resonance, with band intensity and broadening consistent with the increasing heterogeneity of the clusters. The growth kinetics has been fitted to a Langmuir-Hill model. Following these results, a reaction model has been proposed to explain the mechanisms involved in the first stages of Cu clustering.