From the thermodynamic data obtained by ion-transfer voltammetry, we derive here the ionic partition diagram of 5,10,15,20-tetraphenyl-21H,23H-porphine (H(2)TPP) at the water vertical bar 1,2-dichloroethane interface using a simple Born solvation model. This zone diagram shows under which form this porphyrin is present, i.e. neutral, monoprotonated or diprotonated, and in which phase i.e. either in the aqueous or the organic phase as a function of the aqueous pH and the interface polarisation that can be controlled externally or by the distribution of supporting electrolytes. This diagram explains why the monoprotonated form has been difficult to observe when doing biphasic pH titrations. (C) 2010 Elsevier B.V. All rights reserved.