A new scheme of optical film sensor is presented. The sensor is based on p-polarized reflectance, consisting of a sensing coated substrate, is easily optimized for maximum sensitivity in different applications. The resolutions of refractive index m(f), extinction coefficient k(f) and thickness h(f) of the sensitive films are predicted to be 10(-7), 10(-5) and 10(-3) nm, respectively. Experimentally, we selected the sol-gel derived SnO2, films as gas-sensitive films and conducted preliminary gas-sensing test. The results indicate that novel optical film sensor scheme has higher sensitivity, and the detection sensitivity is available to 10(-1) ppm on the condition of optimum optical parameters and incident angle. (C) 2003 Elsevier Ltd. All rights reserved.