Chloroplatinic acid can be incorporated at trace (10(-5)-10(-4) mol dm(-3)) levels into the secondary, hydrated, structure of a single crystal of tetragonal phospho-12-tungstic acid, H3PW12O40.29H(2)O, without any change of the primary structure of the system. During partial reduction of hereropoly tungstate units, mixed-valence W(VI, V) sites are generated; they are capable to mediate the reduction of chloroplatinate to metallic platinum. Consequently, trace amounts of highly dispersed Pt microcenters are produced in the secondary structure of the crystal. Solid-state electrochemical characterization of the material is accomplished with the use of a carbon fiber ultramicrodisk electrode. The system＇s first, most positive, one-electron redox reaction is virtually not affected by traces of Pt in the crystal. By performing voltammetric experiments in two time regimes: we have determined concentration of mixed-valence sites and an effective diffusion coefficient for charge propagation,At more negative potentials, intrastructural discharge of hydronium ions at catalytic Pt microcenters is observed.