A method of reacting a solution comprising a mixture of chemical compounds which are in chemical equilibrium with one another with at least one further chemical compound (9) is provided. The method comprises the following steps: fractionation of the solution by means of a separation method to give at least two fractions (5, 6) which are enriched in different chemical compounds of the mixture; and reaction of a fraction (5) with the further chemical compound or compounds (9). The fractionation is advantageously carried out using a film evaporator (1). Unreacted fractions (6) can be recirculated via a residence time vessel (3) back to the fractionation step. The method is particularly suitable for reactions of an aqueous formaldehyde solution in which various components of the solution (formaldehyde, methylene glycol, polyoxymethylene glycols) react in different ways.