Palladium(0)-catalyzed allylation of uracils in DMSO takes place at N-1 and at N-3. In sharp contrast, regiospecific allylation at N-1 is achieved in an organic-aqueous medium in the presence of palladium(II) acetate and the water-soluble sulfonated triphenylphosphine P(C 6 H 4 -m-SO 3 Na) 3 (tppts). Similar reactions with 2-thiouracils are also drastically dependent on the solvent, taking place at N-1 and N-3 (in dioxane) or at sulfur (organic-aqueous medium). Probably reactions in the organic-aqueous medium are kinetically controlled whereas allylations in DMSO or refluxing dioxane are thermodynamically controlled. © 1990.