For the first time, we measure the molar absorptivity (in the infrared range) of a surfactant species adsorbed onto a solid substrate, epsilon(ads). With this measurement, infrared methods can be used to determine the surface coverage of a surfactant adsorbed at an interface. In particular, the absorptivity is measured for the asymmetric -CH- stretch of the -CH2- moiety in the adsorbed molecules using octadecyltrichlorosilane (OTS) as the model species. A monolayer of OTS is spread onto the air/water interface of a Langmuir balance and compressed to a predetermined surface coverage. The film is then transferred onto the native silicon oxide layer of a silicon substrate using the Langmuir-Blodgett transfer technique, using a transfer ratio of approximately 1, so that the transferred layer＇s surface coverage is equal to the known coverage of the Langmuir monolayer. The infrared spectra of the transferred films are measured, and the molar absorptivity is calculated for the methylene asymmetric stretch in the region of 2920 cm(-1). The epsilon(ads) is found to be independent of surface coverage for the monolayers transferred in the LE, LC, and LE-LC coexistence phases. This absorptivity was found to be 1.7 x 10(6) cm/mol of -CH2-, which is nearly 50% larger than our measured value of the molar absorptivity of the same species in bulk. The absorptivity was found to be dependent on the surface coverage in the LS state. The accurate determination of the surface coverage at the air/water interface necessitates minimizing the polymerization of OTS both at the air/water interface and in the spreading solution. We report a set of parameters for this minimization.