A procedure for simultaneous determination of the thickness and the optical dielectric constant of a thin organic film on a transparent substrate has been proposed. With this procedure these physical quantities can be determined from data collected by a conventional ultraviolet-visible spectrometer without depending on any dispersion model of the organic film. The thickness and the dielectric constant (in the wave number range of 3.6 x 10(4)-1.2 x 10(4) cm(-1)) have been determined for 25-layer and 37-layer Langmuir Blodgett films of a calcium arachidate-arachidic acid mixture. The results for these films agree with each other, showing the utility of the proposed procedure. (c) 2005 Elsevier B.V. All rights reserved.