To avoid the problem of separation of products from catalyst in homogeneous catalysis(1), two-phase systems have been developed in which the catalytic complex (usually a water-soluble organometallic complex) remains in one (generally aqueous) phase while the products remain in a second, immiscible phase(2). Catalysis relies on the transfer of organic substrates into the aqueous catalyst phase; but the limited solubility of these substrates in water leads to reaction rates much lower than those for conventional homogeneous catalysis. Here we show that catalysis at the interface of a two-phase system can be enhanced by using a ’promoter ligand’ which, although soluble in the organic phase, will bind to the organometallic catalyst and thus increase its concentration dose to the interface in the aqueous phase. We demonstrate this approach for the hydroformylation of 1-octene using a rhodium-based catalyst. A rate enhancement by a factor of 10-50 is observed when we introduce the promoter ligand PPh(3) in the organic phase.