The possibility of using a long path length spectroelectrochemical cell to determine the rate constant and diffusion coefficient for a catalytic reaction was explored. An optimization technique combined with a semi-expanding space grid digital simulation was utilized for fitting response curves obtained with the cell to estimate the parameters. The method possesses excellent capability of accurate and precise parameter estimation using synthetic response curves under potential step disturbance, with 0.003 absorbance unit normally distributed noise, for one-electron transfer coupled to a homogeneous catalytic process. Then the technique was used to determine the rate constant of ascorbic acid oxidized by ferricyanide and the diffusion coefficient of ferrocyanide for the system of ferrocyanide-ascorbic acid in glycine-nitric acid solution, which are 6.81 M(-1)s(-1) and 6.68 x 10(-6) cm(2) s(-1), are not sensitive to experimental conditions.