The mechanism of Stille reactions (cross-coupling of ArX with Ar'SnnBu 3)p erformed in the presenceo ffluoride ions is established. At riple role for fluoridei ons is identified from kinetic data on the rate of the reactions of trans-[ArPdBr(PPh 3) 2 ](Ar = Ph, p-(CN)C 6 H 4)w ith Ar'SnBu 3 (Ar' = 2-thiophenyl) in the presence of fluoride ions. Fluoride ions promote the rate-determining transmetallation by formation of trans-[ArPdF(PPh 3) 2 ], which reacts with Ar'SnBu 3 (Ar' = Ph, 2-thiophenyl)a tr oom temperature, in contrast to trans-[ArPdBr(PPh 3) 2 ], which is unreactive. However, the concentration ratio [F À ]/[Ar'SnBu 3 ]m ust not be too high, because of the formationo fu nreactivea nionic stannate [Ar'Sn(F)Bu 3 ] À. This rationalisest he two kinetically antagonistic roles exerted by the fluoridei ons that are observed experimentally, and is found to be in agreement with the kinetic law.I na d-dition, fluoride ions promote reductive elimination from trans-[ArPdAr'(PPh 3) 2 ]g enerated in the transmetallation step.