The performance of several palladium precatalysts, namely, palladium(II) acetate, palladium(O) nanoparticles encapsulated into poly(amidoamine) (PAMAM) dendrimers (Pd DENs), and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(phenyl)stannane and iodoarenes in water is compared. The reactivity of Pd DENs is similar or inferior to that of palladium(II) acetate, although the presence of the dendrimer suppresses the formation of homocoupling products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occurs via palladium species which are leached from the nanoparticle but which remain coordinated to the dendritic macromolecule.