Description: The system NaAlSi 3 O 8 (albite, Ab)-H 2 O offers a simple and tractable model to study the thermodynamics of the volatile constituent H 2 O 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 H 2 O ( \({a_{{H_2}O}}\) ) in hydrous albite liquids and vapors has proven challenging. There are several problems. First, \({a_{{H_2}O}}\) in hydrous liquids relies on melting experiments in the presence of mixed fluids with reduced H 2 O activity (H 2 O-CO 2 and H 2 O-NaCl), but models of \({a_{{H_2}O}}\) in these coexisting fluids have lacked sufficient accuracy. Second, the role of the solubility of albite in H 2 O 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 H 2 O 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 – \({X_{{H_2}O}}\) liquidus relations at 0.5–1.5 GPa with accurate activity formulations for H 2 O in mixed fluids (Aranovich and Newton, 1996, 1999). This yields isobaric T – \({a_{{H_2}O}}\) 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 \({a_{{H_2}O}}\) and P : $$T\left( {{a_{{H_2}O}},P} \right) = {m_0} + {m_1}{a_{{H_2}O}} + {m_2}a_{{H_2}O}^2 + {{m_3}a_{{H_2}O}^3}^\circ C$$ where T is °C, m 0 = 1119.6 + 112.3 P , m 1 =–856.5–578.9P, m 2 = 1004.1 + 952.9 P , and m 3 =–477.1–618.0 P . 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 H 2 O is incorporated into the model to give the correct liquidus H 2 O 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-H 2 O contents and activities from the solubility measurements of Makhluf et al. (2016) in the binary system Ab -H 2 O at 1.0 GPa. The model equation also accurately reproduces the liquidus H 2 O activities from Eggler and Kadik (1979) and Bohlen et al. (1982) when the Aranovich and Newton (1999) activity formulation for CO 2 –H 2 O mixed fluids is applied to their datasets.