The dynamics of charge separation at tin oxide electrodes modified with CdS nanoparticles is investigated using intensity modulated photocurrent spectroscopy. Analytical expressions for the frequency dependent photocurrent, which results from small amplitude modulation of the illumination intensity, are derived. The influence of the applied potential on the rates of charge carrier transfer and recombination is discussed. The experimental data is fitted and rate constants for electron transfer from the nanoparticle to the substrate, hole capture, recombination and back electron transfer are determined. The photocarrier dynamics indicate that the electron remains in the conduction band and that the hole is rapidly trapped. The energy of the trap-state is reported.