Very-thin layers of ionic liquids are formed at microelectrode surfaces during electrolysis of undiluted redox liquids containing only supporting electrolyte at a low concentration. The layers consist of ionic product and counterion coming from supporting electrolyte. Formation of ionic-liquid layer leads to increased viscosity, changes in activity coefficients, and thus to changes in diffusion coefficients of all species involved in the electrode process. This engenders also a specific type of convection. Exchange of one supporting electrolyte for another may change dramatically the physicochemical properties of the generated ionic layer and, in consequence, the magnitude of the electrochemical response of an undiluted substance. A computational model for predicting the phenomena mentioned above is presented. An important step in the model is extension of the concentration dependence of diffusivities to concentrations corresponding to ionic liquids. The calculations done have shown a substantial influence of counterion volume on the electrochemical behavior of undiluted redox liquids even if the counterions are present at a very-low concentration. The consequences of changing supporting electrolyte concentration are also included in the model. The theoretical predictions were compared with experimental data obtained for undiluted nitrobenzene and methanol. The model can be used for both transient and steady-state responses.