This paper presents a comparison of the shapes of the thermal history curves for the rear surface of a sample undergoing laser irradiation. The curves are obtained experimentally and also calculated using the quasi-implicit technique of finite differences. The position and the size of the plateau observed on the theoretical and experimental curves has a significant impact on the accuracy of the determination of the thermal diffusivity obtained using the axial laser flash method. The experimental measurements were performed on phase-change composite materials and also on moist porous material samples (a gypsum slurry).