Size-quantized CdS particles (Q-CdS) capped with 2-aminoethanethiol and 2-mercaptoethanesulfonate were prepared by means of the AOT/heptane inverse micelles method. The resulting CdS particles were covalently immobilized in a high dispersion on an Au(lll) surface coated previously with a self-assembled monolayer of 3,3’-dithiobis(succinimidyl propionate). The immobilized Q-CdS particles were stable against tip scanning in scanning tunneling microscopy (STM). Tunneling spectroscopy (TS) of a single particle whose size was determined from a STM image allowed successfully the determination of the band-gap energy of the size-quantized particle, and the band-gap value obtained was in agreement with that predicted from the tight-binding approximation.