Computer simulation was used to study the evolution of product concentration in electroinjection analysis (EIA) and electrophoretically mediated microanalysis (EMMA). Diffusion of reactants, reverse reaction and commensurability of sample and reagent concentrations were taken into consideration. The results for the case of no reverse reaction and reagent in plenty coincide with the analytical results previously published by the authors. Numerous cases for different values of parameters were simulated and presented in the paper.