Ion-selective electrode using zirconium(IV) complex with octaethylporphin (H2oep) as a carrier showed high selectivity to triphosphate (TP, H5tp) against other hydrophilic anions including diphosphate and phosphate. The electroactive species was identified to be [(Zr4(oep)4(Htp)2] (TP/Zr ratio of 0.5) of the unique structure; triphosphates are recognized by one Zr atom through three O atoms on three different P atoms and by another Zr atom through two O atoms on two terminal P atoms and are also involved in complementary intermolecular hydrogen bonding to be surrounded by four porphyrin complexes. In contrast, Zr(IV) in the other complex with tetraphenylporphin has the higher Lewis acidity, due to the electron-withdrawing property of phenyl rings and, at the higher TP concentration, forms a species having a TP/Zr ratio of unity, which precipitates to lose the electroactivity. The electrode was successfully applied to monitor hydrolysis of TP that provides diphosphate and phosphate.