This paper deals with a study of the hydrogenation of N-substituted aliphatic aldimines, especially in the relation between a heterogeneous hydrogenation catalyst and the formation of by-products. It was found that Pd and Pt catalysts exhibit high selectivity for the hydrogenation of aliphatic aldimines of the type R-1-CH = N-CH-R-2 to the corresponding amine R-1-CH2-NH-CH2-R-2, while the selectivity of Ni and Co catalysts is substantially lower. When hydrogenation is catalysed by Ni and Co, then the predominant products, besides R-1-CH2-NH-CH2-R-2 , are symmetric amines R-1-CH2-NH-CH2-R-1 and R-2-CH2-NH-CH2-R-2. The reaction mixtures also contain R-1-CH = N-CH2-R-1, R-2- CH = N-CH2-R-2 and R-2-CH = N-CH2-R-1 during hydrogenation. Reaction pathways leading to the formation of these by-products were postulated. The first step is Ni- and Co-catalysed isomerisation of the imine R-1-CH = N-CH2-R-2 to R-2-CH = N-CH2-R-1, which occurs under conditions of catalytic hydrogenation, and even at lower temperature when dehydrogenation reactions are excluded (e.g. 50 degrees C). The tautomeric imine reacts further with the original imine, yielding R-1-CH = N-CH2-R-1 and R-2-CH = N-CH2-R-2. It was found that the second reaction occurs without any metal catalyst present; it is reversible and runs until equilibrium is reached. The third step is hydrogenation of both the latter imines to the corresponding symmetric dialkylainines. Step-wise mechanisms for the metal-surface-catalysed imine isomerisation and surface hydrogenation of N-substituted aldimine, which explain differences in the activity of the metals to catalyse the imine isomerisation. were suggested. (C) 2007 Elsevier B.V. All rights reserved.