K+ and Cs+ exchanged X zeolite containing PdCl2 catalyzes the Heck reaction of PhI or PhBr with styrene in toluene or polymethylbenzenes without the need of extrinsic base. The reaction is truly heterogeneous and no leaching was observed. The activity of the fresh catalyst can be regained in a large extent upon reactivation with water washings. The unprecedented preferent formation of the 1,1-diphenylethene has been observed using PdCl2-CsX as catalyst, a fact that has been related to the smaller size of the transition state leading to this isomer compared to stilbene. (C) 2003 Elsevier Science B.V. All rights reserved.