A new photothermal technique is proposed. It is based on the temperature characteristic of the dielectric constant of light‐irradiated material. When chopped light is absorbed in a dielectric material, an alternating variation of capacitance is caused by the heat produced due to light absorption and is detected by using a frequency demodulation technique. A quantitative derivation is presented for the alternating capacitance variation in terms of the optical, thermal, dielectric, and geometric parameters of the system. The experimental results for lead zirconate titanate ceramics, barium titanate single‐crystal, TiO2–Bi2Ti4O11ceramics, and ethyl alcohol are shown, where the observed signals agree with theoretical ones very well. ©1996 American Institute of Physics.