The novel phosphinine-based coordination compound [Cp∗Ir(P^C)(CH3CN)]CF3SO3 (P^C = cyclometalated 2,4,6-triphenylphosphinine) could be synthesized by chloride abstraction from [Cp∗Ir(P^C)Cl] with AgOSO2CF3 and crystallographically characterized. It turned out that this species is the first phosphorus-containing Ir(III) complex which shows a remarkable activity in the cerium ammonium nitrate driven water oxidation reaction. In situ NMR spectroscopic investigations further reveal that water is added selectively to one of the P=C double bonds with formation of four stereoisomers. Moreover, [Cp∗Ir(P^C)] species, possibly OH-functionalized but still having Cp∗ and P^C-ligands contemporary bound to iridium, are present in solution, even under catalytic conditions. (Figure Presented).