To develop a new technology for the purification of formaldehyde from formalin by a distillation method, the theory of vapor liquid equilibrium of the formaldehyde-water binary system was investigated. Based on the studies of Thompson and Bryant9), and Bezzi and Iliceto1), the vapor-liquid equilibria of formaldehyde and water were calculated and the results were in good agreement with the reported data. It was found that the partial pressure of formaldehyde is determined by the free formaldehyde concentration while the partial pressure of water is determined by the mole fraction of free water.