Heck reactions of aryl bromides with cyclohexene and cyclopentene catalyzed by typical homogeneous as well as heterogeneous Pd catalysts (Pd/C, Pd/SiO2, Pd/MgO, Pd/Al2O3, and Pd(0), Pd(II) and [Pd(NH3)(4)](2+) in zeolites Y or ZSM-5) have been studied in order to get detailed information on the reaction mechanism with regard to the catalyst. The focus of the present investigation was on correlations between selectivity (Heck products: double bond isomers of arylcycloalkenes, dehalogenation and double arylation products) and nature of the catalyst or active Pd species, respectively. The results indicate that dissolved molecular Pd species are responsible for the Heck coupling for both homogeneous and heterogeneous (solid) catalysts, whereas dehalogenation is due to a mechanism involving the surface of solid Pd metal particles and radical processes. The selectivity of the reactions can be controlled by the choice of catalyst and reaction conditions (base, solvent, temperature). (C) 2004 Elsevier B.V. All rights reserved.