There is currently considerable interest in nanocrystalline semiconductor electrodes as they offer a material with a large surface area to volume ratio and, if the particles are of sufficiently small dimensions, may display quantum confinement effects. This paper is concerned with impedance and photoelectrochemical investigations of electrodes modified with a sub-monolayer of CdS nanoparticles. Measurements of the integrated admittance and the optical transfer function are reported. From the results obtained the total charge and the Faradaic charge associated with electron injection onto the nanoparticles is determined. It is shown that in a solution of high ionic strength the charge on the nanoparticles is shielded from the underlying substrate. The change in the optical cross-section of the nanoparticles per injected electron is determined by fitting the experimental data using equivalent circuits.