Process for preparing high-concentration formaldehyde solutions having a CH 2 O content of >=50% by weight from an aqueous formaldehyde solution having a lower CH 2 O content by evaporation of part of this solution (partial evaporation), in which the aqueous formaldehyde solution is heated to an evaporation temperature T at which the gas phase becomes enriched in water relative to the liquid phase and the gas phase formed is taken off continuously or discontinuously, wherein the evaporation temperature T obeys the relationship: T[° C.] max [° C.] where T max (c)=A+Bx(c/100)+Cx(c/100) 2 +Dx(c/100) 3 and A=+68.759, B=+124.77, C=-12.851, D=-10.095, where c is the instantaneous CH 2 O content of the aqueous formaldehyde solution during the evaporation in percent by weight and is from 20 to 99% by weight.