Layer-by-layer deposition of polyelectrolyte/metal nanoparticle films in porous alumina, track-etched polycarbonate, and nylon substrates yields catalytic membranes. With all three substrates, scanning electron microcopy images demonstrate a high density of well-separated nanoparticles in the membrane pores. These nanoparticles catalyze the reduction of nitroaromatic compounds by sodium borohydride with rate constants that are the same as those for nanoparticles immobilized on alumina powder. Moreover, the membranes selectively catalyze the reduction of nitro groups in compounds containing other reducible functionalities such as cyano, chloro, and styrenyl moieties. With nitrophenols and nitroanilines, the only reduction product is the corresponding amine. In contrast, nitrobenzene, nitrotoluenes, nitrobenzonitriles, chloronitrobenzenes, and m-nitrostyrene also form a nitroso product. Membrane catalysts are particularly attractive for controlling product distributions through variation of solution fluxes, as demonstrated by the formation of increased levels of nitroso compounds at high flux.