Formaldehyde is one of the most important chemical intermediates. In technical processes, it is used in aqueous solutions, which also contain methanol. These solutions are complex reacting mixtures, in which formaldehyde is bound in a variety of oligornerization products. Distillation experiments with mixtures of formaldehyde, water and methanol were carried out in a 50 mm diameter column with 3.5 m of structured packing. Two types of separations were studied: (a) removal of methanol, for which the bottom product is formaldehyde-water, and (b) removal of formaldehyde, for which the bottom product is pure water. Reflux ratio, pressure, and the feed composition were systematically varied. A physico-chemical model of vapor-liquid equilibria of formaldehyde containing mixtures was used as a basis for a process simulation with an equilibrium stage model. It is assumed that chemical equilibrium is established on each stage. Predictions with that simple model together with stage efficiencies from the internals supplier show good agreement only in some cases and indicate that the influence of reaction kinetics has to be taken explicitly into account. Reactive distillation line diagrams for the ternary system formaldehyde-water-methanol are presented for the first time. (c) 2004 Elsevier B.V. All rights reserved.