The system NaAlSi3O8 (albite, Ab)-H2O offers a simple and tractable model to study the thermodynamics of the volatile constituent H2O in felsic magmas. Although it has been studied in this context for nearly 100 years, developing a comprehensive model that adequately describes the activity of H2O ( aH2O) in hydrous albite liquids and vapors has proven challenging. There are several problems. First, aH2O in hydrous liquids relies on melting experiments in the presence of mixed fluids with reduced H2O activity (H2O-CO2 and H2O-NaCl), but models of aH2O in these coexisting fluids have lacked sufficient accuracy. Second, the role of the solubility of albite in H2O has been assumed to be negligible; however, it is important to take solubility into account at pressure (P) above 0.5 GPa because it becomes sufficiently high that H2O activity at the wet solidus is significantly less than 1. Third, the dry melting temperatures and wet solidus temperatures are inconsistent between the datasets. We address these issues by combining previous experimental work on T– XH2O liquidus relations at 0.5–1.5 GPa with accurate activity formulations for H2O in mixed fluids (Aranovich and Newton, 1996, 1999). This yields isobaric T– aH2O sections at 0.5, 0.7, 1.0 and 1.5 GPa. Data at each isobar were fit to cubic equations, which were used to derive the following equation for liquidus T as a function of aH2O and P:T(aH2O,P)=m0+m1aH2O+m2a2H2O+m3a3H2O°Cwhere T is °C, m 0 = 1119.6 + 112.3P, m 1 =–856.5–578.9P, m 2 = 1004.1 + 952.9P, and m 3 =–477.1–618.0P. The equation is valid at 0.5 < P < 1.5 GPa and T solidus < T < T dry melting. The nonzero solubility of albite in pure H2O is incorporated into the model to give the correct liquidus H2O activity when truncating the model equation in the limiting case where T → T solidus at a given pressure. This model equation reproduces both the liquidus-H2O contents and activities from the solubility measurements of Makhluf et al. (2016) in the binary system Ab-H2O at 1.0 GPa. The model equation also accurately reproduces the liquidus H2O activities from Eggler and Kadik (1979) and Bohlen et al. (1982) when the Aranovich and Newton (1999) activity formulation for CO2–H2O mixed fluids is applied to their datasets.