The kinetics of the reaction of PhPdl(AsPh3)(2) (formed via the fast oxidative addition of Phi with Pd-0(AsPh3)(2)) with a vinyl stannane CH2=CH-Sn(n-Bu)(3) has been investigated in DMF This reaction (usually called transmetalation step) is the prototype of the rate determining second step of the catalytic cycle of Stille reactions. It is established here that the transmetalation proceeds through PhPdl(AsPh3)(DMF), generated by the dissociation of one ligand AsPh3 from PhPdl(ASPh(3))(2)-PhPdl(AsPh3)(DMF) is the reactive species, which leads to styrene through its reaction with CH2=CH-SnBu3. Consequently, in DMF, the overall nucleophilic attack mainly proceeds via a mechanism involving PhPdl(AsPh3)(DMF) as the central reactive complex and not PhPdl(AsPh3)(2). The dimer [Ph2Pd2(mu(2)-l)(2)(AsPh3)(2)] has been independently synthesized and characterized by its X-ray structure. In DMF, this dimer dissociates quantitatively into PhPd](AsPh3)(DMF), which reacts with CH2=CH-SnBu3. The rate constant for the reaction of PhPdl(AsPh3)(DMF) with CH2=CH-SnBu3 has been determined in DMF for each situation and was found to be comparable.