The efficiency of electron injection from photoexcited dye molecules into TiO2 nanoparticles with two different sizes (20 or 300 nm) was studied using the time-resolved microwave conductivity (TRMC) technique. For the ruthenium dye (N719) adsorbed on 20 nm TiO2 particles, we have found a near 100% injection efficiency, which is consistent with previous transient optical absorption studies. In contrast, the electron injection efficiency for N719 sensitized on 300 nm TiO2 particles was found to be less than 50%. A similar behavior was found for coumarine-sensitized TiO2 nanoparticle films. The difference is tentatively attributed to differences in the adsorption of dye molecules on the TiO2 surface. Interestingly, only for 20 nm TiO2 particles, a trap filling effect has been observed, that is, at low illumination intensities, the TRMC signal increases more than proportional with the illumination intensity. This is attributed to an increase in electron mobility due to occupation of deep traps. The absence of a significant trap filling effect for 300 nm particles points toward a considerably lower trap density.