Results are reported from uv-vis absorption measurements in the diffusion layers of planar, cylindrical and spherical electrodes. The reversible one-electron oxidation/reduction processes of N,N,N’, N’-tetramethyl-p-phenylendiamine have been used as an example. Diffusion coefficients and formal rate constants for the heterogeneous electron transfer were evaluated from time resolved absorbance vs. distance profiles. Digital simulation was used to fit experimental absorbance profiles as function of the rate parameters. Heterogeneous rate constants were determined at platinum and at mercury electrodes. The results are compared with values obtained by other electrochemical methods.