In this paper are presented the general equations for a model of competitive facilitated ion transfer reactions across oil I water interfaces. In other words, the transfer of two cations M-1(z1)+(w) and M-2(z2)+(w) that, under certain conditions, compete for a neutral ligand is analyzed. The results obtained agree with those of Matsuda and coworkers (J. Electroanal. Chem. 509 (2001) 128) for the same system under limiting conditions and also with the general voltammetric criteria reported by Girault et al. (J. Electroanal. Chem. 449 (1998) 49, J. Electroanal. Chem. 451 (1998) 59) for the facilitated transfer of a single cation assisted by a neutral ligand. The equations reported in this paper allow the cyclic voltammetric response of the system to be simulated under all the possible conditions. The results are analyzed for different zones determined by the cation/ligand concentration ratios. The principal contribution of this work is the complete analysis and characterization of a competitive transfer mechanism of two different cations. The first cation transfer is followed by an interfacial cation-exchange (TICE) reaction in accord with the following equation: M-1(z1)+(w) + M2Lz2+(o) reversible arrow M1Lz1+(o) + M-2(z2)+(w). The experimental conditions where a cation is transferred by two different mechanisms during the same potential sweep are defined (e.g. aqueous complexation followed by transfer (ACT) and TICE mechanism or transfer by interfacial complexation (TIC) and TICE mechanism).