Films of AOT-capped (AOT = dioctyl sulfosuccinate) cadmium sulfide nanoparticles (Q-CdS) prepared by incorporation into a self-assembled monolayer (SAM) of hexanethiol on Au were prepared. These were imaged at negative substrate bias in air by scanning tunneling microscopy, and Q-CdS particles were shown to cover most of the area on the SAM. Continuous scanning of the tip over the substrate removed Q-CdS particles to the outside of the scanning area because of tip-substrate interactions. Scanning tunneling spectroscopy was carried out in air with the tip held over the thin CdS film (100 nm) and over individual Q-CdS particles in the layer. The i vs V and di/dV vs V curves indicated that the energy band gap of the Q-CdS particles is wider than that of the thin CdS films. The results of photoelectrochemistry also indicated that the onset photopotential is more negative for smaller Q-CdS particles and is related to the level of conduction band.