The stable oxygen isotope composition of orthophosphate (δ18OPO4) is a widely used (paleo)temperature indicator and more recently, a useful tracer of phosphorus-cycling. In natural aqueous systems (e.g., oceans, rivers, soil/ground water) the largest reactive phosphorus pool is dissolved inorganic phosphate. Here, we present a new experimentally-determined equation for thermodynamic equilibrium O-isotope fractionations between dissolved phosphate and water, catalyzed by the enzyme inorganic pyrophosphatase (PPase) between 3 and 37°C;1000lnα(PO4-H2O)=14.43(±0.39)1000/T(K)-26.54(±1.33)r2=0.99The new equation is slightly offset by +0.5 to +0.7‰ from recent empirically-determined fractionations based on biogenic apatite, with both based on modern cf-irms TC/EA analysis of Ag3PO4. Dissolved phosphate–water fractionations are offset by +0.9 to +2.3‰ from the earlier empirical relation for biogenic phosphate–water fractionation determined using fluorination of BiPO4. The equation presented here is thus, specific to equilibrium fractions between dissolved phosphate and water and appropriate for use in recent/future oxygen isotope studies of dissolved phosphate using similar cf-irms TC/EA analytical methods.