We plan to investigate by in situ XAS a photocatalyst for H2 evolution composed, in the as-formed state, of an amorphous sputtered NiCu thin co-catalyst film on a mesoporous TiO2 layer (Fig. 1a). We propose that the active catalytic sites for H2 evolution are formed in situ by (i) dissolution of the NiCu thin film in the dark in the aqueous reaction phase, followed by (ii) Ni and Cu ions re-deposition on TiO2 upon UV illumination. The self-assembled NiCu sites provide efficient and stable photocatalytic H2 evolution. In situ XAS will allow to understand the mechanism of self-assembly of this photocatalytic system and the nature of the active sites for H2 evolution.