This paper presents for the first time detailed NMR investigations of the changes of the palladium coordination sphere for dissolved Pd salts (Pd(OAc)2) and chosen solid, heterogeneous catalysts (Pd(II)/Al 2O3, Pd(II)/NaY zeolite) under Heck reaction conditions for the interesting model substrates bromobenzene and chlorobenzene (and styrene as olefin) using catalytic amounts of Pd only. The information obtained from the mechanistic studies led us to new optimized simple "ligand free" catalyst systems and reaction conditions that allow the activation of chlorobenzene in an unexpected manner under relatively mild reaction conditions.