A process for preparing acrylic acid or methacrylic acid comprises (a) preparing a gaseous product mixture having essentially the composition of a reaction mixture of the catalytic gas phase oxidation of C3-/C4-alkanes, -alkenes, -alkanols and/or -alkanals and/or precursors thereof to form acrylic acid or methacrylic acid, characterized by (b) condensing said product mixture, (c) crystallizing acrylic acid or methacrylic acid from the solution obtained in step (b), and (d) removing the resulting crystals from the mother liquor.