Ultrasmall TiO2 particles (2R = 9.0 +/- 0.5 Angstrom) were generated in situ in a water-in-oil (w/o) microemulsion composed of water, cetyldimethylbenzylammonium chloride (CDBA), and benzene, by controlled hydrolysis of TiCl4. Effective photosensitization of the TiO2 particles by Ru(II) tris(bipyridine), Ru(bpy)(3)(2+), proceeds in the w/o microemulsion. The photosensitization of TiO2 was studied by following the emission decay of excited RU(bpy)(3)(2+) at different TiO2 concentrations. The emission decay curves of excited Ru(bpy)(3)(2+) were analyzed by assuming a Poisson distribution of the TiO2 particles over the reverse micelles and realizing that excited Ru(bpy)(3)(2+) photosensitizes TiO2 by two photosensitization pathways : (a) intramicellar electron injection quenching (k(q)) and (b) particle incorporation by intermicellar exchange (k(c)) followed by fast intramicellar quenching. The derived values of intramicellar electron injection and intermicellar exchange rate constants were kg = 7 X 10(6) s(-1) and 1.4 x 10(9) M(-1) s(-1), respectively. The kinetic analysis allowed to estimate the mean agglomeration number of the TiO2 particles to be 11 +/- 2, which corresponds to a particle diameter of 9.0 +/- 0.5 Angstrom, Effects of the water-pool size on the TiO2 particle structure and photosensitization process were also investigated. As the water-pool size increases, the intermicellar exchange rate constant is enhanced but the intramicellar electron injection rate is unaffected. The agglomeration number is higher in larger water pools.