Al3+ species when incorporated by impregnation into a H-Y zeolite together with the Pt species significantly increases the yield of branched paraffins, Al, Zn and Cd species although very different in chemical nature from each other, all show a significantly effect of isomerization enhancing or cracking suppression at almost the same molar concentration. Such behavior is ascribed to the formation of new desorption-transfer promoting sites which can rapidly remove the carbocationic intermediates, decreasing their residence times on the acidic sites and thus preventing any further cracking. A triangular 'zeolite acid/Pt dehydro-hydrogenating/Al desorption-transfer promoting' site configuration is hypothesised. (C) 2000 Elsevier Science B.V. All rights reserved.