The present paper presents a new experimental method developed for obtaining, simultaneously, the thermal conductivity and thermal diffusivity of nonmetallic materials. The method is based on signal treatment in the frequency domain. Two fast, high sensitivity heat flux transducers and two surface thermocouples measure the heat flux and temperatures, respectively. Time dependent temperature and heat flux signals at the boundaries define the equivalent complex impedanceZ. Fourier transform is applied to these signals and the impedanceZis obtained in the frequency domain. The difference between the experimental values and the values ofZestimated using a theoretical model is then minimized. The result is the simultaneous estimation of the thermal diffusivity and thermal conductivity. The use of both the modulus and the phase of an equivalent complex impedance promotes a separation of parameters, which assures the obtained values of &lgr; and &agr; to be unique. Error analysis and a comparison with the steady‐state method are also presented. ©1995 American Institute of Physics.