A thermodynamic model for phase and chemical equilibrium in formaldehyde - water mixtures was extended to describe vapor-liquid and chemical equilibrium in the binary formaldehyde-methanol system and in the ternary formaldehyde- water-methanol system. Ternary mixtures were extended using new experimental vapor-liquid equilibrium data for the influence of especially small amounts of methanol on the volatility (of small amounts) of formaldehyde in aqueous solutions. New experimental data for the range 333 to 383 K are reported. Some model parameters were fitted to ternary phase equilibrium data, because of cross-reactions between poly(oxymethylene) glycols and poly(oxymethylene) hemiformals, and strong influence of intel actions between those species on the phase equilibrium of the ternary mixture. The new model presents a reliable method for calculating vapor-liquid equilibrium and the conversion of formaldehyde to reaction products [poly(oxymethylene) glycols and poly(oxymethylene) hemiformals] over the whole range of temperature and composition where reliable experimental data are available.