A DFT study of R-R reductive elimination (R = Me, Ph, vinyl) in plausible intermediates of Pd-catalyzed processes is reported. These include the square-planar tetracoordinated systems cis-[PdR2(PMe3)(2)] themselves, possible intermediates cis-[PdR2(PMe3)L] formed in solution or upon addition of coupling promoters (L = acetonitrile, ethylene, maleic anhydride (ma)), and tricoordinated intermediates cis-[PdR2(PMe3)] (represented as L = empty). The activation energy ranges from 0.6 to 28.6 kcal/mol in the gas phase, increasing in the order vinyl-vinyl < Ph-Ph < Me-Me, depending on R, and ma < "empty" < ethylene < PMe3 approximate to MeCN, depending on L. The effect of added olefins was studied for a series of olefins, providing the following order of activation energy: p-benzoquinone < ma < trans-1,2-dicyanoethylene < 3,5-dimethylcyclopent-1-ene < 2,5-dihydrofuran < ethylene < trans-2-butene. Comparison of the calculated energies with experimental data for the coupling of cis-[PdMe2(PPh3)(2)] in the presence of additives (PPh3, p-benzoquinone, ma, trans-1,2-dicyanoethylene, 2,5-dihydrofuran, and 1-hexene) reveals that: (1) There is no universal coupling mechanism. (2) The coupling mechanism calculated for cis-[PdMe2(PMe3)(2)] is direct, but PPh3 retards the coupling for cis-[PdMe2(PPh3)(2)], and DFT calculations support a switch of the coupling mechanism to dissociative for PPh3. (3) Additives that would provide intermediates with coupling activation energies higher than a dissociative mechanism (e.g., common olefins) produce no effect on coupling. (4) Olefins with electron-withdrawing substituents facilitate the coupling through cis-[PdMe2(PR3)(olefin)] intermediates with much lower activation energies than the starting complex or a tricoordinated intermediate. Practical consequences are discussed.