The thermal diffusivity of yttria-zirconia (Y2O3-ZrO2) single crystals, with YO1.5 concentrations in the range of 0-60 mol%, has been determined using a new method that is based on spatially resolved (20 mu m) infrared mapping of a modulated thermal field. The decreasing trend of the thermal conductivity (K), as a function of the YO1.5 content (up to 20 mol%), can be described using a model based on a Debye approach that has been modified hg introducing a cut-off length for the phonon mean free path. At higher concentrations, K increases, as a result of a possible ordering of the point defects.