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A method of measuring the phase lag of the photoacoustic signal obtained by rear surface excitation is employed for the determination of the thermal properties of insulating dental materials such as calcium hydroxide cement, a cavitary varnish and photopolymerized restoration resins. Disk shaped dental materials were glued to the front surface of the thin metal foil window of the photoacoustic cell and illuminated by pulsed light. A three layer model was developed to fit the measurements of the phase angle as a function of the frequency of excitation. The thermal diffusivity and conductivity of the dental material were determined to good precision. The instrumentation employed in the present work is simple and easily obtainable by any laboratory. © 1998 American Institute of Physics.