The vapor-liquid equilibria in the quaternary system of formaldehyde(1)-methanol(2)-methylal(3)-water(4) were measured experimentally at a pressure of 101.33 +/- 0.02 kPa, and at temperatures from about 318 to 367 K. The concentration ranges of the liquid phase were, formaldehyde < 15% (in mass), methanol < 42%, and methylal and water from 0 to 100%. Brandani’s physico-chemical model was extended to correlate the experimental VLE data. The UNIQUAC binary mono-parameter for the interaction between the pair of formaldehyde and methylal, xi(13), and the Henry law constant of formaldehyde in mixed solvents, H-FMS, were obtained by data correlation. The calculated vapor compositions deviate from the measured values by 0.008 (mole fraction) for formaldehyde, 0.010 for methanol, 0.018 for methylal, and 0.019 for water. The mean deviation of temperatures is 1.5 K.