A process and an apparatus for the post-purification in particular of pyrolysis gases for preparing pure formaldehyde, hemiformal being pyrolyzed in an evaporator, the resulting formaldehyde vapor being passed through a condensed liquid component collected in the bottom area of the evaporator, the condensed liquid component being passed out as a falling film situated on the inner wall through a cooled outlet tube disposed in the bottom area of the evaporator and the formaldehyde vapor being passed through the cooled outlet tube in cocurrent and the liquid component and the formaldehyde being taken off separately at the outlet tube end remote from the evaporator.