Methylal (also known as dimethoxymethane and formaldehyde dimethyl acetal) is a side product of the production of polyacetal plastics from formaldehyde, which has to be removed in downstream processing. The design of the separation equipment requires a verified model for the vapor-liquid-phase equilibrium. New experimental results are presented for the vapor-liquid and liquid-liquid equilibrium of the ternary system (formaldehyde + water + methylal) and for the vapor-liquid equilibrium of the ternary system (formaldehyde + methanol + methylal) and of the quaternary system (formaldehyde + water + methanol + methylal). New experimental results and the literature data are compared with prediction results.