Under a pressure of syngas in pyridine, RhCl(PPh3)3 selectively catalyzes the dimerization of enolizable C(n) aldehydes, possessing one methylene group in the alpha-position to the aldehyde function, into saturated dimers, C2n monoaldehydes. The yields are good to moderate. The pressure and relative composition of syngas, together with the use of a triphenylphosphine-ligated metal complex are crucial for obtaining good selectivities. Monosubstitution of the alpha-carbon severely limits the dimerization. Alcohols and aromatic aldehydes do not react. A tentative reaction mechanism is proposed in which water is involved. The dimer aldehydes can subsequently be oxidized in situ by air or various oxidants such as m-chloroperbenzoic acid or t-butylhydroperoxide into the corresponding nor-ketones. This process constitutes a novel access to such ketones.