In the sepn. of (meth)acrylic acid (I) from mixts. contg. (I) as main prod. and lower aldehydes as by-prods. by rectification in a column with a distn. zone and a concn. zone, the mixt. is not supplied to the column directly. Instead, it is passed to a heated dwell vessel connected to the concn. zone, in which the mixt. is kept boiling, and the sump liquid from this vessel is fed to the column.