We have investigated a model thin film sensory system in which analytes diffuse into multilayers of a fluorescent conjugated polymer. The film thickness is precisely controlled by depositing discrete monolayers by the Langmuir-Blodgett technique. The effects of analyte mass transport and energy migration on the photophysical properties of the films were investigated by conducting UV-vis, fluorescence, and electrical conductivity measurements. Thin films show different properties when compared to relatively thick films due to prevailing surface phenomena. The diffusion constant of the analyte through the films is estimated to be similar to7 x 10(-14) cm(2)/s from an analysis of a phenomenological model. A bilayer LB film exposed to the analyte implies higher sensitivity in fluorescence quenching compared to a solution system due to a fast interpolymer energy migration in the condensed phase. However, as the number of layer increases, the efficiency of fluorescence quenching decreases. The difference between a sensory system with emissive surface traps and one with bulk distributed quenching traps is discussed.