In this work, we report on the interdiffusion of Ge and Si in Ti3SiC2 and Ti3GeC2, as well as that of Nb and Ti in Ti2AlC and Nb2AlC. The interdiffusion coefficient, D-int, measured by analyzing the diffusion profiles of Si and Ge obtained when Ti3SiC2-Ti3GeC2 diffusion couples are annealed in the 1473-1773 K temperature range at the Matano interface composition (approximate to Ti3Ge0.5Si0.5C2), was found to be given by D-int (m(2)/s) = (0.3 +/- 0.2) exp -350 +/- 3 (kJ/mol) / RT D-int increased with increasing Ge composition. At the highest temperatures, diffusion was halted after a short time, apparently by the formation of a diffusion barrier of TiC. Similarly, the interdiffusion of Ti and Nb in Ti2AlC-Nb2AlC couples was measured in the 1723-1873 K temperature range. The D-int for the Matano interface composition, viz. approximate to(Ti-0.5,Nb-0.5)(2)AlC, was found to be given by D-int (m(2)/s) = (12 +/- 3) exp -350 +/- 3 (kJ/mol) / RT At 1773 K, the diffusivity of the transition metal atoms was approximate to 7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter.