A very general theory is derived for the voltarnmetry of two polarizable interface (2PI) systems in the presence of an arbitrary number and concentration of transferable species in the two aqueous phases. Analytical equations are derived for the current response in any voltammetric technique, regardless of the charge number and li-pophilicity of the ionic species. The theory covers the situation where the compensating ion is not in excess, which gives rise to notable distortions of the signal as compared with the most usual case where it is assumed in a large excess. The case where the transfer of a single transferable ion is compensated by more than one species is also considered. This situation can lead to multiple signals in the voltammogram that can be misinterpreted as the presence of several analytes. The theoretical results are validated experimentally by studying the transfer of the cation tetraethylarnmonium in water vertical bar 1,2-dichloroethane vertical bar water cells in presence of several compensating ions of different li-pophilicity and concentration.