The formation mechanisms of metal particles (platinum (Pt) particles) in an aqueous ethanol solution of poly(AL vinyl-2-pyrrolidone) (PVP) by the photoreduction method have been studied by transmission electron microscopy (TEM) and in situ and ex situ X-ray absorption fine structure (XAFS) analysis. The average diameter of the dilute and concentrated Pt particles in the PVP solution is estimated from TEM to be 2.0 and 2.5 nm, respectively. XAFS analysis was performed for the reduction process of pt(4+) ions to metallic Pt particles for the Pt L-3 edge of the colloidal dispersions of the concentrated Pt solutions. The photoreduction process proceeds by the following steps: (1)reduction of PtCl62- to PtCl42-, (2) dissociation of Cl from PtCl42-, followed by of reduction of Pt2(divided by) ionic species to Pt-0. (3) formation of a Pt-0-Pt-0 bond and particle growth by the association of Pt-0-Pt-0. The reduction f PtCl42- to pt(0) is a slower process, compared with the reduction of PtCl62- to PO42-. There is a delay between the disappearance of PtCl42- and the formation of Pt-0-Pt-0 clusters.