The diffusion coefficient of carbon in TiC1-y and ZrC1-y has been determined as a function of stoichiometry and temperatur e in the range between 1200 and 1750°C. Use has been made of diffusion couples of the type metal/pure carbon as well as (metal+carbide)/(carbon+carbide). From marker experiments in the latter type of couples we found that carbon is virtually the only diffusing component. Since the partial molar volume of carbon in these carbides is practically zero, the intrinsic diffusion coefficient of carbon is equal to the interdiffusion coefficient D~. Very accurate carbon analyses have been performed using EPMA, in diffusion couples as well as in the carbides present in equilibrated alloys. These enabled us to determine accurately the homogeneity region of the carbides in this temperature range and provided concentration profiles, from which diffusion coefficients could be found. From these profiles it is immediately clear that the diffusivity of carbon is a function of the carbon concentration: the diffusion coefficient increases with decreasing carbon concentration. Comparing the diffusion behaviour of carbon in TiC1-yand ZrC1-y we found, that both the concentration and the temperature dependence were the same. If the difference in melting point is taken into account, the interdiffusion coefficients in TiC1-y and ZrC1-y as a function of the homologous temperature T/Tm are nearly identical. The expressions for the interdiffusion coefficients are: D(TiC1-y) = [0.48exp(9.2y)] * exp[-(39500/T)] cm²/s D(ZrC1-y) = [0.0083 exp(9.2y)]exp[-(36600/T)] cm²/s In the literatur e a number of results have been published, often conflicting and not mentioning the large diffrences in diffusion coefficients up to a factor of 100 originating from differences in the carbon content in the carbides. Nearly all reported values fit within the range found by us as a function of composition and temperature.