The equilibrium conditions governing isothermal, isobaric phase equilibrium in aqueous two-phase systems containing ionic solutes are derived from basic thermodynamics assuming that each of the coexisting phases is electrically neutral. For nonionic species the result reduces to the common equilibrium condition, i.e. the equal activity criterion. For ionic species the equal activity criterion does not hold, and has to be modified. However, like the activity, the modification can be calculated from an expression for the Gibbs energy of the uncharged mixture. Usually the concept of electrical potential difference is used to express the phase equilibrium for ionic components. It is proved that this concept only provides an alternative way of expressing phase equilibria. However, the new derivation gives an explanation for this electrical potential difference and allows its calculation.