Dielectric dispersion measurements have been carried out on barium titanate single crystals of different thicknesses at temperatures above the Curie point ranging up to 180°C. In the high‐frequency range of the measurements (100 kc), the measured dielectric constant of thin crystals is consistently much smaller than that of thick samples. The effect is ascribed to the presence of a layer of low dielectric constant on the surface of the crystals, with a capacitance of the order of 0.5 &mgr;f/cm2in the temperature range explored. The relaxation time of this surface layer is found to be at least 10−4sec at 120°C on crystals which have not been ``aged'' recently.