Models to describe/predict the phase equilibrium of aqueous solutions of carboxylic acids are of considerable importance to the chemical industry. Such acids (e.g. adipic acid, citric acid, acrylic acid) are obtained as solids during the production process and/or are purified by a combination of distillation and crystallization (i.e. evaporative crystallization). Therefore, for the computer-aided design of such processes, a thermodynamic model for the solid-liquid phase equilibrium is required which should also be able to describe the vapor-liquid equilibrium reliably. The present publication deals with the industrially important system methacrylic acid-acetic acid-water. Experimental data for the solid-liquid phase equilibrium are presented and compared to model predictions. The model takes into account physical interactions (through the UNIQUAC equation for the Gibbs excess energy) as well as the complexing due to hydrogen bonding (through chemical reaction equilibria). The acids are assumed to exist only as dimers (of the same carboxylic acid monomers) and cross-dimers (of two different acid monomers) in the liquid and solid phases. The model parameters were fitted to experimental data for the liquid-solid equilibrium of the binary subsystems only. The ternary solid-liquid phase equilibrium and the binary as well as the ternary liquid-liquid and the vapor-liquid phase equilibria were predicted and compared to experimental data. That comparison reveals a good agreement. Thus, the model provides a reliable basis for the computer-aided design of a downstream process which combines distillation and crystallization. (C) 2003 Elsevier Science B.V. All rights reserved.