This study investigates the electrocatalytic water oxidation activity of manganese phosphates, specifically focusing on the structure-performance relationship. Two homologous catalysts, KMnPO4 and KMnPO4·H2O, with 4-coordinated and 6-coordinated Mn centers, respectively, were synthesized. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species during water oxidation was identified, with Mn(V)=O species being crucial for O–O bond formation. In KMnPO4, the Mn coordination structure changed from 4-coordinated [MnO4] to 5-coordinated [MnO5], which displayed a triangular biconical configuration. This structure flexibility favored the retention of Mn(III)–OH and the generation of Mn(V)=O, leading to higher intrinsic activity. In KMnPO4·H2O, the Mn coordination structure remained stable, but the Mn(V)=O species was at equilibrium with Mn(IV)=O, with the concentration of Mn(IV)=O determining the intrinsic activity. The study provides a clear understanding of the water oxidation mechanism in Mn-based systems.This study investigates the electrocatalytic water oxidation activity of manganese phosphates, specifically focusing on the structure-performance relationship. Two homologous catalysts, KMnPO4 and KMnPO4·H2O, with 4-coordinated and 6-coordinated Mn centers, respectively, were synthesized. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species during water oxidation was identified, with Mn(V)=O species being crucial for O–O bond formation. In KMnPO4, the Mn coordination structure changed from 4-coordinated [MnO4] to 5-coordinated [MnO5], which displayed a triangular biconical configuration. This structure flexibility favored the retention of Mn(III)–OH and the generation of Mn(V)=O, leading to higher intrinsic activity. In KMnPO4·H2O, the Mn coordination structure remained stable, but the Mn(V)=O species was at equilibrium with Mn(IV)=O, with the concentration of Mn(IV)=O determining the intrinsic activity. The study provides a clear understanding of the water oxidation mechanism in Mn-based systems.