In this study, a thermodynamic model was combined with the conductor-like screening model for real solvents (COSMO-RS), coupled with chemical reaction equilibrium and mass balance, for describing the phase behavior and complex chemical equilibrium in the ternary system of formaldehyde water-methanol at temperatures between 323 and 383 K. To the best of our knowledge, this is the first time accurate and reasonable predictions were made for the species distribution and vapor-liquid equilibrium (VLE). Meanwhile, the sigma-profiles and sigma-potential were employed to interpret the molecular interaction in the system. Compared to the extrapolation method, COSMOtherm implementation was preferred here for the prediction of the vapor pressures of methylene glycol (MG) and hemiformal (HF). Furthermore, the maximum partition coefficient for formaldehyde in the dilute formaldehyde water methanol mixture at low temperature was discussed. (C) 2016 Elsevier B.V. All rights reserved.