Gold nanocrystals were formed and grown in simultaneously polymerizing hybrid organic–inorganic films. For the preparation of Au containing films, tetraethyl orthosilicate and methacryloxypropyltrimethoxysilane were separately hydrolyzed and the resulting sols were mixed, followed by the addition of a photoinitiator and a NaAuCl4 solution in methanol. The resulting solutions were spin-coated onto glass-substrates, and the so-formed films were irradiated with a solar simulator at powers ranging from 200 to 800 W. The irradiation resulted in simultaneous polymerization of the films and formation of gold nanoparticles. The irradiated films were characterized by x-ray diffraction measurements, ultraviolet–visible optical absorption spectroscopy and transmission electron microscopy studies. After irradiation at 800 W, the transmission electron microscopy experiments showed the presence of homogeneously distributed Au nanoparticles with a size distribution ranging from 2 to 12 nm. The interpretation of the results indicates that the Au particle growth depends on the matrix polymerization rate; enhancing the rate by increasing the irradiation power or the photoinitiator concentration results in smaller particle domains. This result is explained referring to influence of the polymerization rate on the diffusion of gold species through the host.