The photodeposition process of Rh metal nanoparticles on a TiO2 photocatalyst from RhCl3 aqueous solution in the presence of methanol as a sacrificial oxidant, which consists of the direct reduction of Rh3+ ions to Rh metal and the formation of Rh nanoparticles, was uncovered by in situ time-resolved energy-dispersive X-ray absorption fine structure (DXAFS) analysis in a liquid-solid suspension state. The fractions of Rh metal particles and Rh3+ precursor were estimated by the least-squares fitting of each X-ray absorption near-edge structure (XANES) spectrum by a linear combination of authentic spectra corresponding to Rh and Rh3+ The fraction of Rh metal linearly increased with photoirradiation time and saturated after 90 min of photoirradiation. The coordination number (Rh-Rh pair) was evaluated by the curve fitting of the Rh-Rh scattering at 2.45 angstrom in the Fourier transforms (FT) of extended XAFS (EXAFS) spectra. The coordination number linearly increased with photoirradiation time and attained a constant value of 10 after 90 min of photoirradiation. This value is lower than that for the Rh foil (12). These suggest the formation of fine Rh metal nanoparticles on TiO2. In addition, the diminution rate of Rh3+ as determined by ICP analysis was in good agreement with the increased rates for the fraction of Rh metal particles estimated by XANES spectra and the coordination number (Rh-Rh pair) evaluated by FT-EXAFS spectra. This result strongly supports the fact that electrons generated by charge separation reduce the Rh3+ precursor to an Rh metal particle at a moment in time and at a constant rate. The Rh particles do not grow in incremental steps, but Rh particles with a uniform size appear one after another on the surface.