The geochemical behaviors of phosphate-containing species at mineral surfaces are of fundamental importance for controlling phosphorus (P) mobility, fate, and bioavailability. Understanding these interfacial behaviors in water-unsaturated environments, where minerals are covered by thin water films, is of special importance in the context of soil vadose zone geochemistry. This study resolved the transformation of pyrophosphate to orthophosphate within the confines of nanometer-thick water films condensed on nanosized birnessite (MnO2). Time-resolved vibrational spectroscopy indicated that PP was effectively degraded into monodentate mononuclear complexes on MnO2 and at rates that followed zero-order kinetics. We found a direct link between PP degradation kinetics and water film thickness, which revealed the role of sorbed water as the key nucleophilic agent needed for P-O-P bond rupture. By highlighting the essential roles of surface complexation and water loadings in controlling pyrophosphate transformation, this work adds new insight needed to advance knowledge on the global cycling of P in nature. It can also have broader applications to catalysis and energy storage solutions involving MnO2 nanoparticles.