We estimated the water fugacity (f H2O) in the trachytic magma of Phlegrean Fields using the biotite–magnetite–sanidine equilibrium. We confirm that the partly ionic model is the most appropriate to estimate annite activity (aannite) for the experimental biotite. Crystallization experiments were carried out on a representative sample of trachytic Breccia Museo eruption, Naples, Italy. Experiments were performed in the temperature and pressure range of 725–870 °C and 50–200 MPa, and redox conditions ranging from NNO (nickel–nickel oxide) + 1 (±0.2) to FMQ (fayalite + magnetite + quartz). Most experiments were done under conditions of H2O saturation (Pwater = Ptotal). Few experiments were done using a mixed H2O–CO2 fluid phase. The pre-eruptive fH2O of the Phlegrean Fields magma is estimated at 735 bars, which compares well with available melt inclusion constraints. Our results confirm that this geohygrometer can be used in any magmatic systems bearing the worked assemblage.