The thermal conductivities of Y2O3, Y2O3:Nd3+, Y3Al5O12:Nd3+, and LaF3have been measured from 77°–300°K by a steady‐heat‐flow method. The densities of these crystals and of LaF3:Nd3+have been determined in the same temperature range, using x‐ray methods. Using this information and specific heat data from a variety of sources, the thermal diffusivities of these materials have been calculated. Although the diffusivity of pure yttrium oxide is somewhat higher than that of pure yttrium aluminum garnet, introduction of about 1 mole % of neodymium into both these compounds leads to crystals with diffusivities approximating that of undoped YAG. The diffusivity of doped yttrium oxide and YAG (about 0.05–0.06 cm2/sec at room temperature) is about three times that found for undoped lanthanum fluoride. The relative ease with which heat may be removed from Nd‐doped Y2O3and YAG suggests their use in lasers to diminish time variations in the width of the beam.