In an aqueous formaldehyde solution comprising formaldehyde in the form of monomeric formaldehyde, methylene glycol and polyoxymethylene glycols in a total concentration x of .gtoreq.65% by weight, the mean molar mass {overscore (M)} of the polyoxymethylene glycols is, as a function of the formaldehyde concentration, equal to or less than the values given by equation I:.times..times..times..times..times..times..times..times..times- ..times..times..times..times..times..times..times..times..times..times..ti- mes..times..times..times..times..times..times..times..times..times. ##EQU00001## where: {overscore (M)} is the mean molar mass, and x is the total concentration of formaldehyde in the form of monomeric formaldehyde, methylene glycol and polyoxymethylene glycols in % by weight (total formaldehyde concentration).