The activity and selectivity of several catalysts based on Ni(4,4'-bipyridine)Cl-2 complex co-activated with AlCl(C2H5)(2) and supported on different molecular sieves such as Y, L, mordenite, mesoporous MCM-41 and on amorphous silica-alumina utilised in the dimerization of ethylene at 298 K and initial pressure of the monomer 12 atm is presented. Chemo- and stereo-selectivity effects expressed as selectivity for dimers and oligomers, and respectively, the concentration Of 1-C4H8 in C4H8 fraction and the molar ratios 2-trans-C4H8/2-cis-C4H8 are discussed. The results are correlated with the steric and diffusion hindrances induced by the micro- and meso-porous structure respectively, of the porous solid matrix. (C) 2004 Elsevier B.V. All rights reserved.