Thermal diffusivity (D) as a function of temperature (T) was determined from common and brittle micas both parallel and perpendicular to the basal plane using laser-flash analysis, which does not suffer from problems with contact losses and radiative gains. Samples were characterized using the electron microprobe and visible spectroscopy. For the two orientations, D differs by almost a factor of 10, such that for flow within the cleavage plates D is ∼2 mm2s–1, similar to many silicate minerals. Thermal diffusivity decreases as F or Fe contents increase. Data are best described as D = FT–G + HT, where F, G, and H are fitting parameters. Thermal conductivity up to 1000 K is calculated from D, density, and heat capacity for muscovite, phlogopite, and biotite. The pressure dependence of k is inferred from recent data from muscovite for flow across the basal plane.