A process for the production of high molecular weight .alpha., .beta.-unsaturated aldehydes by alkylation of the .gamma.-carbon atom of a lower molecular weight .alpha., .beta.-unsaturated aldehyde with a substituted allyl alcohol which has a chain branching in the 3-position carbon atom or a phenyl radical in the 3-position or of a corresponding allyl alcohol carboxylic ester at elevated temperature in the liquid phase. The products are valuable intermediates for the production of natural substances and odorants.