An improved numerical way of solving the second Fick’s law for the purpose of modelling chronoamperometry of uncharged species in a solution containing an insufficient amount of supporting electrolyte is presented. The simulation of the mixed diffusional and migrational transport is based on the Crank-Nicolson method with an exponentially expanded space grid. The simulation scheme includes the electron transfer rate and assumes the use of hemispherical microelectrodes. The electroneutrality condition was not explicitly imposed to the calculation model but deviations from this principle were controlled and found to be negligible for the transport. The scheme that has been worked out allows the computation of changes in the concentration profiles and the electrical variables (current, true electrode potential, resistance and ohmic drop) as functions of time and support ratio, c(supp.el.)/c(substrate). The results obtained demonstrate that the true potential of the working electrode in a solution with deficit of supporting electrolyte differs significantly from the applied potential and changes in time. This makes chronoamperometry resemble voltammetry. The standard heterogeneous rate constant of the electron transfer strongly influences the way the current changes in time and therefore can be determined from chronoamperometric experiments.