Evidences of the passivation effect are given when thin films of CdS are deposited on GaSb crystalline substrates, using a bath chemical deposition method. The passivation process is studied through photoacoustic and photoluminescence experiments. The surface recombination velocity calculated from photoacoustic measurements decreases and the radiative recombination rate as measured from photoluminescence spectra increases when the nominal S/Cd ratio in the layer deposition solution increases. The influence of the CdS layer thickness on the surface passivation of GaSb is also studied.