Sildenafil (Viagra(TM)) was examined for its ionization and lipophilicity by two-phase titration and electrochemistry at the interface between two immiscible electrolyte solutions (ITIES) in the 12-dichloroethane/H2O system. The dissociation constants (basic pK(a) = 6.78, acidic pK(a) = 9.12) and partition coefficients of the various species, together with the effects of electrical potential, were used to construct an ionic partition diagram (pH-potential representation). This allowed to interpret the transfer mechanisms of sildenafil at liquid/liquid interfaces, suggesting in particular that an intramolecular H-bond influences the lipophilicity of the neutral and cationic species. Conformational calculations confirmed this hypothesis