Photo-activation of semiconductor films for room temperature gas sensing is a research topic intensely studied during the last years. Even though the most investigated semiconductors in the field of conductometric gas sensors are metal oxides, cadmium sulfide shows very interesting photo-stimulated properties, in particular it shows chemoresistivity that can be activated by light at room temperature. In this work, the photo-enhanced sensing properties of CdS films toward five target gases are studied as a function of the wavelength of the impinging radiation. It resulted that the chemical activity of CdS at surface is strongly dependent on the exciting wavelength. The peak of the response was found for the wavelength equivalent to the bandgap energy, this meaning a bandgap-resonant behavior of the surface chemical activity that can possibly be employed for gas sensing applications.