New experimental results are reported for the solubility of ammonia in liquid mixtures of (water + methanol). Five (gas-free) solvent compositions are considered (mole fraction of methanol of about 0.05, 0.25, 0.5, 0.75, and 0.95). The temperature amounts to about 313 K, 353 K, and 393 K. The solubility pressure ranges up to about 0.1 MPa, 0.6 MPa, and 1.6 MPa, respectively. The molality of ammonia in the solvent mixture of (water + methanol) (the mole fraction of ammonia in the liquid) ranges up to 13 mol kg(-1) (about 0.3). The experimental results are used to determine Henry's constant of ammonia in the liquid mixtures of (water + methanol). An extension of Pitzer's molality scale-based model for the Gibbs excess energy (to solvent mixtures) is applied in a thermodynamic model to describe the phase equilibrium. (C) 2007 Elsevier B.V. All rights reserved.