Investigating the interfaces between electrolytes and electrocatalysts during electrochemical water oxidation is of great importance for an understanding of the factors influencing catalytic activity and stability. Here, the interaction of a well-established, nanocrystalline and mesoporous Ca-birnessite catalyst material (initial composition K_0.2Ca_0.21MnO_2.21·1.4H₂O, initial Mn-oxidation state ∼+3.8) with an aqueous potassium phosphate buffer electrolyte at pH 7 was studied mainly by using various electron microscopy and X-ray spectroscopy techniques. In comparison to electrolyte solutions not containing phosphate, the investigated 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⁻/O²⁻ by H_nPO₄⁽³⁻ⁿ⁾⁻ were observed, leading to the formation of a stable, partially disordered Ca–K–Mn–H_nPO₄–H₂O layer on the outer and the pore surfaces of the active electrocatalyst material. In this surface layer, Mn³⁺ 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 the soft Ca 2p and Ca L-edge X-ray spectroscopy. A possible way to interpret the observed, obviously very favorable “special relationship” between (hydrogen)phosphates and Ca-birnessites in electrocatalytic water oxidation would be that H_nPO₄⁽³⁻ⁿ⁾⁻ anions are incorporated into the catalyst material where they act as stabilizing units for Mn³⁺ highly active centers and also as “internal bases” for the protons released during the water-oxidation reaction.