An experimental technique to measure simultaneously the thermal diffusivity and thermal conductivity of small specimens of thermal‐insulator materials is described. Experimental conditions are developed which approach the mathematical idealization of a one‐dimensional, infinite, two‐component, composite solid whose planar interface is subjected to a step‐function heat flux. The heat flux is generated by resistive heating of a thin metallic foil. Although this principle has been exploited previously, the present technique permits specimens with volumes two orders of magnitude smaller than prior work based on the same idealization. A number of difficulties related to small specimen volume are identified and overcome. Another improvement over past efforts is the use of the nonlinear least‐squares analysis to determine the optimum values for diffusivity and conductivity. A comparison of measurements of these properties for acrylic and Pyrex glass with literature values suggests that the technique affords accuracies of about 5% for materials in this range.