DE3047485

Methanol is allowed to flow downwardly and contacted countercurrently with an unsaturated aldehyde-containing gas obtained from gas phase catalytic oxidation of the starting gas containing at least one gas selected from the group consisting of propylene, isobutylene and t-butanol, said methanol being supplied in such an amount that substantially the whole thereof can be gasified in the treatment. According to this method, the water content in said unsaturated aldehyde-containing gas can be rendered smaller than that in the azeotropic mixture of the unsaturated aldehyde and water, and also there takes place no polymerization of the unsaturated aldehyde.