Process for preparing high-concentration gaseous formaldehyde having a molar CH.sub.2O:H.sub.2O ratio of .gtoreq.0.6 from an aqueous formaldehyde solution by evaporation of at least part of the solution, in which the aqueous formaldehyde solution is heated to a vaporization temperature T and the gas phase formed is taken off, wherein the evaporation temperature T obeys the relationship: T[.degree. C.].gtoreq.T'.sub.min[.degree. C.] where T'.sub.min(c)=A+B.times.(c/100)+C.times.(c/100).sup.2+D.times.(c/100).sup- .3 and A=+68.759, B=+124.77, C=-12.851, D=-10.095, where c is the instantaneous CH.sub.2O content of the aqueous formaldehyde solution during the evaporation in percent by weight and is from 20 to 99% by weight.