The electronic structure of an n-CdS / electrolyte interface was analyzed by in situ surface conductance for CdS thin films obtained by chemical precipitation onto a conducting substrate. The films are compact, composed of crystallites and exhibit grain boundaries that affect the potential distribution on the semiconductor space charge layer. We propose an explicit evaluation of the charge carrier density, n(L) taking into account the relation between the space charge layer length, L-SC, and the crystallite length, L. Using this model, values of the interfacial and electronic parameters, such as the flat band potential, U-fb, the carrier mobility, , and the electron density, n(o), were obtained from in situ surface conductance data.