Ion-transfer reactions of a water-soluble porphyrin, meso-tetrakis(p-sulfonatophenyl)porphyrin (H2TPPS4-), and its zinc complex (ZnTPPS4-) at the nitrobenzene \water (NB \ W) interface were studied by cyclic voltammetry. In particular, for H2TPPS4-, the effect of acid-base equilibria on the ion-transfer potential was analyzed theoretically. In addition, the photocurrent response for ZnTPPS4- at the NB \ W interface was observed potentiostatically, and a certain effect of EDTA as the sacrificial reagent on the photocurrent was confirmed.