Heck reactions were performed with 4-bromoacetophenone and n-butyl acrylate, yielding the trans-arylated acrylate ester with high selectivity. Pd-zeolite catalysts were compared with supported Pd metal catalysts. In order to obtain an active and heterogeneous catalyst, the preformed or in situ-formed catalytically active Pd-0 species and the Pd-II intermediates must be sufficiently stabilized on the zeolites against leaching and against aggregation to inactive Pd black precipitates. The different states of Pd in the zeolites, i.e., Pd(NH3)(4)(2+) , lattice-coordinated Pd ions, PdO particles, and Pd-0 metal particles of different sizes, were studied by a combination of diffuse reflectance UV-vis, XRD, SEM, and TEM before, during, and after catalytic reactions. The heterogeneous nature of the catalysis largely depends on the catalyst pretreatment and on the solvent and base used. With tributylamine as the base and in toluene, the Heck olefinations with Pd(NH3)(4)(2+) -zeolites (0.4 wt% Pd; mordenite, Y, ZSM-5) and Pd-0-mordenite (0.4 and 4 wt% Pd) are truly heterogeneous. Pd leaching from the zeolites, as evaluated in a very strict filtrate activity test, is clearly related to the presence of oxidized Pd-II in an all-oxygen environment, i.e., ionic Pd 2 or PdO. Supported Pd catalysts (Pd/C and Pd/SiO2) give rise to Pd leaching as well. The heterogeneous reactions with the zeolites can be accelerated by the addition of a quaternary ammonium salt promoter. In addition, the heterogeneous Heck catalysts can be applied in a continuous reactor.