The dielectric parameters of Western Oil /shale from three sites were measured to assist in the design of an in situ RF dielectric heating oil recovery process. The dielectric parameter measurements were made under simulated in situ conditions; and these showed that, while the parameters vary with site and grade, the dielectric absorption deceased rapidly as the temperature rises above 200 C and drops to a more or less constant plateau up ;to about 450 C. The dielectric breakdown occurred at 420 C for RF heating rates of approximately 63 W/Kg or greater. The pronounced drop in dielectric absorption was correlated with the decomposition of an oil shale component and the evolution of bound water. Above 200 C the dielectric absorption is cause by the mineral constituents, since the dielectric parameters did not vary with production of oil and gas.