A novel trace gas detection scheme based upon the photothermal effect following light absorption is described. The resulting index change, which is proportional to the trace species absorption and concentration, is measured interferometrically in a stabilized Fabry–Perot cavity. An experimental noise limit corresponding to an absorption coefficient of 4×10−8cm−1/( &sqrt;Hz) was observed. We discuss possible improvements and estimate the ultimate sensitivities achievable with this technique.