The Heck reaction between aryl halides ArX (X = Br, Cl) and n-butyl acrylate was studied in the presence of catalyst systems containing Pd(OAc) 2 and a tertiary phosphine ligand (PR 3 ,R 2 P-(CH 2 )-PR 2 ). It is shown that all tested arylphosphines except P(o-Tol) 3 and P(Mes) 3 undergo an extensive PC bond cleavage at temperatures higher than 120°C. As a consequence, vinylic side products are formed with concomitant catalyst deactivation. In the case of triarylphosphines side products originate from reaction intermediates of the type Pd I I (PAr 3 ) 2 (Ar)X. These species undergo an aryl-aryl exchange between the palladium center and coordinated phosphine ligands. Subsequent intermolecular phosphine scrambling leads to several isomerized arylpalladium species, all of which couple with n-butyl acrylate to give the corresponding cinnamic ester side products.