New experimental data are presented that allows the biotite–magnetite–sanidine equilibrium to be used for estimating water fugacity (fH2O) in hydrous phonolitic magmas. It is also demonstrated that the partly ionic model gives the best estimate for the annite activity (aannite). Crystallization experiments were carried out on a representative sample of peralkaline, phonolitic obsidian of Montaña Blanca (MB) pumice deposit, Tenerife, Canary Islands. Experiments were performed from 720–810 °C and 50–250 MPa. Redox conditions were varied between NNO (nickel + nickel oxide) + 1 (±0.2) and FMQ (fayalite + magnetite + quartz). The majority of the experiments were done under H2O saturation conditions (Pwater = Ptotal). Several experiments were done using a mixed H2O-CO2 fluid phase whereas in other experiments 10 or 20 wt% powdered alkali feldspar was added to the starting material. The pre-eruptive fH2O of the Montaña Blanca magma is estimated at 676 ± 200 bars. The pre-eruptive fH2O for the Fish Canyon tuff (753–2978 bars) and Bishop tuff rhyolite (1065–2440 bars) were also calculated, as well as fH2O for metamorphic biotite from Au Sable Forks (≈ 130 bars). The results of this study suggest that this geohygrometer can be used in any magmatic system in which biotite–magnetite–sanidine is a stable assemblage.