This paper presented a revised model for calculating the chemical and vapor liquid equilibria (VLE) of the formaldehyde + water system. Chemical equilibrium constants for the formation of poly(oxymethylene) glycols (HO(CH2O)(i)H, MG(nu) i > 1) were reviewed, and selected models were modified to reliably predict the species distribution of formaldehyde aqueous solutions and describe the influence of overall formaldehyde concentration. Besides, the UNIFAC equation for the Gibbs excess energy was combined with the chemical reaction equilibria to represent the experimental VLE data in the literature. In addition, some model parameters were redetermined from the corresponding data fits.