A new description of the vapor phase of formaldehyde solutions containing methanol is presented. In contrast with all previous models for these mixtures, it is recognized that the dimer resulting from the formaldehyde-methanol reaction has a relatively high volatility and should be considered present in the vapor phase. The new model is matched to the experimental vapor-liquid equilibrium data for binary and ternary mixtures containing formaldehyde, methanol, and water. The proposed model represents well vapor-liquid equilibrium of these reacting mixtures and does not reveal systematic deviations for the ternary system as observed with previous models.