Investigating the interfaces between electrolytes and electrocatalysts during electrochemical water oxidation is of tremendous importance for an understanding of the factors influencing catalytic activity and stability. Here, the interaction of a wellestablished, nanocrystalline and mesoporous Ca-birnessite catalyst material (initial composition K0.2Ca0.21MnO2.21·1.4 H2O, initial Mn-Oxidation state ~+3.8) with an aqueous potassium phosphate buffer electrolyte at pH 7 was studied by using various electron microscopy and spectroscopy techniques. In comparison to electrolyte solutions not containing phosphate, Ca-birnessite electrodes show especially high and stable oxygen evolution activity in phosphate buffer. During electrolysis, partial ion substitutions of Ca2+ by K + and OH- / O 2- by HnPO4 (3-n)- were observed, leading to the formation of a stable, partially disordered Ca-K-Mn-HnPO4-H2O layer on the outer and the pore surfaces of the electrocatalyst. In this surface layer, Mn(III) ions are stabilized, which are often assumed to be of key importance for oxygen evolution catalysis. Furthermore, evidence for the formation of [Ca/PO4/H2O]- complexes located between the [MnO6] layers of the birnessite was found using Ca 2p and Ca L-edge the soft X-ray synchrotron-based spectroscopy. A possible way to interpret the obviously very favorable, “special relationship” between (hydrogen)phosphates and Ca-birnessites in electrocatalytic water oxidation would be that HnPO4 (3-n)- anions are incorporated into the catalyst material where they act as stabilizing units for Mn3+ centers and also as “internal bases” for the protons released during the reaction.