The insertion of the previously coordinated second molecule of propene into the Pd-C bond is the rate-determining step in the dimerization of propene with the Pd(acac)(2) + BF3OEt2 catalyst system. Tetracoordinated square planar Pd hydrides bearing two vacant coordination sites are likely to be catalytically active species in the dimerization of propene. The simplicity of the catalyst system composition might be of industrial importance.