Vapor sensors using thin porous silicon (PS) Fabry-Perot films were prepared and characterized. The detection method used in this work involves measurement of the intensity of reflected light from a microporous Si film as a function of analyte concentration. Analyte adsorption within the pores of the film causes the Fabry-Perot fringes to shift to higher wavelengths as a result of an increase in the average refractive index of the PS layer. Two transduction methodologies are employed: measurement of the intensity of reflected light using a low-power red diode laser source, and measurement of the spectrum of reflected light in the wavelength range 400-1000 nm, using a white light (tungsten) source. The effect of PS film thickness and porosity on sensitivity are systematically studied. A detection limit of 250 ppb for the analyte ethanol in a nitrogen gas carrier stream has been demonstrated. Experimental results suggest that capillary condensation is in part responsible for the high sensitivity of these vapor sensors.