.alpha.-Alkylacroleins are prepared by a process wherein an alkanal is reacted with formaldehyde in the presence of a secondary amine and in the presence or absence, of an acid, in the liquid phase, under superatmospheric pressure and at above 150.degree. C., for not more than 25 minutes.The .alpha.-alkylacroleins obtainable by the process are useful starting materials for dyes, drugs and pesticides.