A combination of chronoamperometric and voltammetric responses at microdisk and conventional size disk electrodes respectively, was used to determine the diffusion coefficient of an electroactive species and the ultramicroelectrode diameter simultaneously. The method does not require knowledge of the bulk concentration and requires only the number of electrons participating in the electrode reaction and a value for the area of the conventional size disk electrode. Subsequent determination of the concentration of electroactive species is also possible. This approach is demonstrated by evaluating the diffusion coefficient of N,N,N’,N’-tetramethyl-p-phenylendiamine (TMPD) in pure and binary mixtures of aprotic solvents in perchlorate media. Parallel determinations were carried out from potentiodynamic I-E curves obtained for ultramicroelectrodes of different radii and corrected for ideal steady state conditions. The diffusion coefficients thus determined were in good agreement with those determined using other electrochemical techniques.