A process for preparing acrylic acid from formaldehyde and acetic acid, is performed by (i) providing a gaseous stream S1 comprising formaldehyde, acetic acid, oxygen and water, where the formaldehyde content of stream S1 is in the range from 8% to 18% by volume, based on the total volume of stream S1, the ratio of the volumes of acetic acid to formaldehyde is in the range from 0.6:1 to 1.1:1, and the molar ratio of oxygen to the total amount of organic carbon is in the range from 0.02:1 to 0.15:1; and (ii) contacting stream S1 with an aldol condensation catalyst in a reaction zone to obtain a gaseous stream S2 containing acrylic acid.