The in situ regioselective hydrogenation of alpha,beta-unsaturated aldehydes using [Ir(COD)Cl](2) stabilized by hydro(pyrazolyl)borate ligands such as: hydrotris(pyrazolyl)borate (Tp), hydrotris(3,5-dimethylpyrazolyl)borate (Tp*), hydrotris(3,4,5-trimethylpyrazolyl)borate (Tp(3Me)) and dihydrobis(3,4,5-trimethylpyrazolyl)borate (Bp*) is described. When using traps-cinnamaldehyde (CNA) as a model substrate, the results show that selectivity towards the saturated aldehydes and the unsaturated alcohol depends on the reaction conditions and becomes more pronounced with the type of stabilizing ligands employed. As an example, when the Tp and Bp* ligands are used, the selectivity shifts to the saturated aldehydes, while for Tp* and Tp(3Me), the selectivity changes dramatically to the corresponding allylic alcohol. The foregoing observation was extended to another alpha,beta-unsaturated aldehyde such as a-methyl-traps-cinnamaldehyde, where the corresponding allylic alcohol was obtained when Tp* was employed as a stabilizing ligand. (C) 2003 Elsevier B.V. All rights reserved.