The transfer of ions at the water 1,2-dichloroethane interface mediated either by erythromycin or by its decomposition products in the presence and absence of a phospholipid monolayer was analysed by de cyclic voltammetry and electrochemical impedance measurements in a four-electrode system. The composition of the liquid phases has an important effect on the transfer processes. Significant differences in their parameters were found depending on the phase in which the antibiotic was initially present, i.e. erythromycin as a base in the organic phase or protonated erythromycin in the aqueous phase. A dependence on pH was also noticed. These results suggest that the ion transfer mechanisms can be modified by the experimental conditions. The charge transfer resistance (R(alpha)) for erythromycin and for its hydrolysis products was measured with and without the phospholipid monolayer. Except when erythromycin was in the organic phase, a general increase in R(alpha), more important for the hydrolysed antibiotic, was observed, indicating that the loss of the sugar units or the opening of the lactone ring hinders the transport process at the phospholipid modified interface.