In an aqueous formaldehyde solution comprising formaldehyde in the form of monomeric formaldehyde, methylene glycol and polyoxymethylene glycols in a total concentration x of ≧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: ( M _ g / mol ) = 48 + 6.589 · 10 - 1 · ( x % ⁢   ⁢ by ⁢   ⁢ weight ) + 4.725 · 10 2 ·   ⁢ ( x % ⁢   ⁢ by ⁢   ⁢ weight ) 2 - 3.434 · 10 - 3 · ( x % ⁢   ⁢ by ⁢   ⁢ weight ) 3 +   ⁢ 9.625 · 10 - 5 · ( x % ⁢   ⁢ by ⁢   ⁢ weight ) 4 - 1.172 · 10 6 ·   ⁢ ( x % ⁢   ⁢ by ⁢   ⁢ weight ) 5 + 5.357 · 10 - 9 · ( x % ⁢   ⁢ by ⁢   ⁢ weight ) 6 ( I ) 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).