A method using eigenfunction expansions has been implemented to determine the diffusion coefficients of electroactive species. The method has been successfully applied to the process of single-electron oxidation of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD). The results obtained by the method agree very well with those from the well established finite difference algorithm and are more reliable in general than the conventional Levich equation. The computer code developed is versatile and efficient; the method could be of wide application.