Both the optical absorption coefficient &agr;, and the carrier densityN, of semiconducting thin films are analytically investigated in relation with their deposition substrate influence. It is shown that these two properties are sensitive to the film thicknessd, particularly when incident monochromatic light is very close to the film transmission cutoff wavelength. &agr; behaves according to the &agr;≊f(d−1) type of function whileNobeys the lawN≊f(d−m) whose precise form varies withmvalue and depends on the substrate influence. Experimental results report on sprayed CdS films respectively deposited on glass, SnO2, and ITO substrates which illustrate the suitability of the model proposed. The film absorption coefficient is affected by the migration phenomenon at the film‐substrate interface, the rate of the migration depending upon the substrate involved.m≊1 obtained with SnO2andm≊2 with ITO indicate different CdS film interface properties, typical of the substrate used. ©1995 American Institute of Physics.