The thermal diffusivity of Teflon, sodium chloride, quartz, and silica glass was measured at 40¿C to pressures of 35, 18, 30, and 36 kbar, respectively. A transient line method was modified for use in a piston-cylinder high-pressure cell. Pressure gradients were determined by experiments with bismuth foils. The pressure dependence of the thermal diffusivity at 40¿C for the substances studied may be represented as follows (k in square centimeters per second, P in kilobars): for the low-pressure phases of Teflon, Teflon I-II, P<5.5 kbar, k=0.0012+3.6¿10-5P; for the high-prssure phase, Teflon III, 5.5 kbar<P<35 kbar, k=0.0012+8.0¿10-5P; for polycrystalline halite, P<18 kbar, k=0.0031+9.5¿10-4P; for quartz, perpendicular to the c axis, P<30 kbar, k=0.031+5.3¿10-4P; for silica glass, P<36 kbar, k=0.0068-6.7¿10-6P. The diffusivity of silica glass decreases with pressure, in contrast to the diffusivity of its crystalline counterpart, quartz, which increases with pressure. In addition to the diffusivity the thermal conductivity of Telfon was determined by measuring the power applied to the heater wire. The thermal conductivity of a Teflon I-II mixture is approximately constant at 0.0075-0.0078 cal/cm s ¿K to 5.5 kbar. Above 5.5 kbar the conductivity of Teflon III is given by K=0.0062+4.0¿10-5P. The specific heat of Teflon decreases with pressure and decreases discontinuously by 15% across the Teflon II-III phase change, in good agreement with the decrease predicted from thermal expansion and compressibility data.