The possibility of a quantitative IR spectroscopic determination of the mass or thickness of Langmuir-Blodgett (LB) layers directly deposited onto a multiple internal reflection (MIR) element has been investigated. The thickness is calculated from the absorbance (band area) ratios obtained for the LB layer and for a bulk layer of either comparable or a much greater thickness. The basic relations describing the absorbance for different kinds of partial covering of a trapezoidic MIR element are summarized: for the double-sided deposition using the LB technique, for the one-sided deposition of a thin layer achieved by dropping a solution, and for the one-sided deposition of a thick layer. The experiments were performed with octadecyl methacrylate on ZnSe and Ge. A relation is given for the number of reflections for the two-sided covering of the reflection element, typical for the LB technique. The investigations covered all bands in the range from 700 to 2930 cm(-1). The set of equations for the absorbances is supposed to be helpful also for the evaluation of other quantitative analysis using the MIR technique.