Formation mechanisms of metal particles (gold (Au) particles) in an aqueous ethanol solution of HAuCl4 with poly(N-vinyl-2-pyrrolidone) (PVP) by the photoreduction method were investigated by UV-vis, transmission electron microscopy (TEM), and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameters of the dilute and concentrated Au particles in PVP solution are estimated from TEM to be 106 angstrom and 925 angstrom, respectively. XAFS analysis was carried out to elucidate the reduction process of AuCl4- ionic species to metallic Au particles for the Au-L-3 edge of the colloidal dispersions of the concentrated Au solutions. In the photoreduction process, the reduction of AuCl2- species to Au-0 atoms is a slower process than that of AuCl4- to AuCl2-, and the reduction of AuCl2- to Au-0 atoms and the association of Au-0 atoms to form seed Au particles (particle diameter between 5.5 and 30 angstrom) concurrently proceeds in the short-duration photoirradiation. In addition, in the long-duration photoirradiation, the slow progression of Au particle growth occurs with the association of Au-0-Au-0 metallic bonds, resulting in the formation of larger Au particles (particle diameter larger than 500 angstrom).