We report that Oxidative addition of bromobenzene to Pd ((PBu3)-Bu-t)(2) Occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: ((PBu3)-Bu-t)Pd (Ph)(Br) approximate to (Hp(t)Bu(3))Br < [((PBu3)-Bu-t)Pd(P-Br)](2) < ((PBu3)-Bu-t)(2)Pd (H)(Br). Studies on the reactions of Pd((PBu3)-Bu-t)(2) in the Presence of ((PBu3)-Bu-t)(2)Pd(H)(Br) showed that the concentration of ((PBu3)-Bu-t)(2)Pd(H)(Br) decreased only after the Pd(O) complex had been consumed. These data indicated that the catalyst in this Process is ((PBu3)-Bu-t)(2)Pd(H)(Br). Thermal decomposition of the three-coordinate Oxidative addition product ((PBu3)-Bu-t)(2)Pd(Ar)(Br) during the reaction of Pd((PBu3)-Bu-t)(2) and bromoarenes ultimately leads to formation Of ((PBu3)-Bu-t)(2)Pd(H)(Br). Parallel reactions of bromobenzene with ((PBu3)-Bu-t)(2)Pd(H)(Br) and Pd((PBu3)-Bu-t)(2) showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(O) species. We therefore propose a catalytic cycle for oxidative addition in which PBu3t center dot HBr reacts with the Pd(O) species to form ((PBu3)-Bu-t)(2)Pd(H)(Br), and ((PBu3)-Bu-t)(2)Pd (H)(Br) reacts with the bromoarene, possibly though the anionic species [(HPBu3+)-Bu-t][((PBu3)-Bu-t)Pd(Br)(-)], to form [Pd((PBu3)-Bu-t)(Ar)(Br)].