The diffusion of strontium and cesium through thin layers (100 μm) of isotropic pyrolytic carbon has been measured over the temperature range 1000 to 1700°C. Diffusion coefficients observed for cesium are orders of magnitude lower than those for strontium. The diffusion rates for both series are much lower than those observed in porous poly crystalline graphite, where little difference is seen between cesium and strontium. When constant chemical potential sources are used, chemical- and self-diffusion measurements for strontium give identical results in the Sr concentration range 0.01 to 0.2 wt%. There is apparently no concentration effect for cesium over the range 1.0 × 10−5 to 0.15 wt%. The difference between strontium and cesium diffusion in pyrolytic carbon is attributed to the greater steric effect of the pyrolytic-carbon defect structure relative to cesium.