The so far accepted mechanism of the Stille reaction (palladium-catalyzed cross-coupling of organotin reagents with organic electrophiles) is criticized. Based on kinetic studies on catalytic reactions, and on reactions with isolated intermediates, a corrected mechanism is proposed. The couplings between (RI)-I-1 (1) (R-1 = C-6- Cl2F3 3,5-dichlorotrifluorophenyl) and (RSnBu3)-Sn-2 (R-2 = CH=CH2, 2a; C6H4-4-OCH3, 2b), catalyzed by trans-[(PdRI)-I-1(AsPh3)(2)] (3a), give R-1-R-2 and obey a first-order law, r(obs) = a[3a][2a]/(b + [AsPh3]), with a (2.31 +/- 0.09) x 10(-5) s(-1) and b = (6.9 +/- 0.3) x 10(-4) mol L-1, for [1] [2a] = 0-0.2 mol L-1, [3a] = 0-0.02 mol L-1, and [AsPh3] = 0-0.07 mol L-1, at 322.6 K in THF, The only organopalladium(II) intermediate detected under catalytic conditions is 3a. The apparent activation parameters found for the coupling of 1 with 2a support an associative transmetalation step (Delta H-obs(double dagger) = 50 +/- 2 kJ mol(-1), Delta S-obs(double dagger) = -155 +/- 7 J K-1 mol(-1) in THF; and Delta H-obs(double dagger) = 70.0 +/- 1.7 kJ mol(-1), Delta S-obs(double dagger) = -104 +/- 6 J K-l mol(-1) in chlorobenzene, with [1](0) = [2](0) = 0.2 mol L-1, [3a] = 0.01 mol L-1). The reactions of 2a with isolated trans-[PdR1 X(AsPh3)(2)] (X = halide) show rates Cl > Br > I. From these observations, the following mechanism is proposed: Oxidative addition of (RX)-X-1 to PdLn, gives cis-[(PdRXL2)-X-1], which isomerizes rapidly to trans-[(PdRXL2)-X-1]. This trans complex reacts with the organotin compound following a S-E(2)(cyclic) mechanism, with release of AsPh3 (which explains the retarding effect of the addition of L), to give a bridged intermediate [(PdRL)-L-1(mu-X)(mu-R-2)SnBu3]. In other words, an L-for-R-2 substitution on the palladium leads R-2 and R-1 to mutually cis positions. From there the elimination of XSnBu3 yields a three-coordinate species cis-[(PdRRL)-R-1-L-2], which readily gives the coupling product R-1-R-2.