Composite films made of polyaniline and cadmium sulfide were prepared by electrochemical polymerization of aniline in an acidic solution in the presence of suspended CdS particles. The resulting composite films exhibited photo-induced electrochromism by illumination at open circuit in the presence of EDTA as an electron donor. Illumination of deprotonated polyaniline/CdS composite films through a mask in the presence of EDTA, however, resulted in the reduction of the polyaniline beyond the illuminated area, being in marked contrast to the photoreduction of TiO2/polyaniline composite films. It is concluded that the proton uptake properties of the semiconductor particles used in the composite film formation with polyaniline plays a key role in determining the resolution of photoreduced areas of polyaniline.