Process for preparing high-concentration formaldehyde solutions having a CH.sub.2O content of .gtoreq.50% by weight from an aqueous formaldehyde solution having a lower CH.sub.2O 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[.degree. C.]