The reaction kinetics of the rhodium-phosphine catalysed hydrogenation of methyl-(Z)-alpha-acetamidocinnamate (MAC) was studied both in micellar solutions and in methanol. A comparison shows a strong dependence of the reaction order of the substrate on the nature of the reaction media. In aqueous micellar solutions of a nonionic and an ionic surfactant the reaction rate gets zero order towards the substrate concentration, due to the stabilisation of the rhodium-substrate complex in the micelles. The reaction rate is only affected by the hydrogen insertion step. The effective activation energies give additional information about the reaction mechanism. (C) 2003 Elsevier Science B.V. All rights reserved.