The formation of supported Pt nanoparticles during the reduction of pt(4+) on TiO2 under H-2 (4% in He) gas flow has been monitored using a differential approach to time-resolved pair distribution function (PDF) analysis of high-energy X-ray scattering data. The reduction of Pt4+ species was found to follow pseudo-zero-order reaction kinetics, with an activation energy of 0.52 eV. Reaction data obtained at constant temperature (100, 150, and 200 degrees C) and during heating (0-200 degrees C) indicate that the growth of nanoparticles from the reduce Pt-0 species depends strongly on temperature and proceeds more rapidly upon heating.