Diffusion coefficients of Al26and Mn54in aluminum have been determined between 450° and 650°C. Low specific activities of the isotopes necessitated use of a thick layer technique. The exact solution to Fick's second law for the appropriate boundary conditions was used in treating the data. Temperature dependence of the diffusion coefficients may be expressed by the following equations:DAl26=1.71 exp−(34 000/RT)DMn54=0.22 exp−(28 800/RT).