The coabsorption system in which both species Red and Ox absorb at the same wavelength was investigated by the long path length spectroelectrochemical technique. The variation of the system absorbance at the isosbestic point with time, which is strongly dependent upon the diffusion coefficients and their difference, was calculated by employing the method of digital simulation under semi-infinite condition. Simulated results indicated that each A(t)-t curve corresponded to definite values of D(R) and D(O) on the condition that D(R) is not equal to D(O). The species diffusion coefficients were evaluated by comparing the spectroelectrochemical experimental result at the isosbestic point with the simulated result, and experimentally verified by the study of electrooxidation of hydroquinone.