The present invention to a process for preparing 2-alkenals of the formula I in which R1 is selected from hydrogen and C1-C4-alkyl; and R2 is selected from hydrogen, C1-C12-alkyl, C2-C12-alkenyl, C4-C8-cycloalkyl and C6-C10-aryl, wherein C1-C12-alkyl and C1-C12-alkenyl may be substituted with C5-C7-cycloalkyl or C5-C7-cylcoalkenyl; comprising dehydrogenating an alkenol of the formula II, an alkenol of the formula III or a mixture thereof, wherein R1 and R2 are each as defined above, wherein the alkenol II, the alkenol III or a mixture thereof is brought into contact with a catalytic system comprising at least one ligand and a metal compound selected from ruthenium(II) compounds and iridium(I) compounds, and wherein the hydrogen formed during the dehydrogenation is removed from the reaction mixture by: v) reaction with a reoxidant selected from C3-C12-alkanones, C4-C9-cycoalkanones, benzaldehyde and mixtures thereof; and/or vi) purely physical means.