The selective hydrogenation of cinnamaldehyde by a RhCl(TPPTS)(3) aqueous solution, ex situ synthesised from RhCl3 (.) nH(2)O and TPPTS (tris(m-sulphonatophenyl)phosphine) with pH control by NaOH solution, was investigated under a biphasic (water/toluene) system in a batch reactor. The hydrogenation more usually occurred at C==C bonds, giving hydrocinnamaldehyde as the main product. Important parameters were varied carefully in order to maximise the selectivity toward hydrocinnamaldchyde for which the highest selectivity of 99.9% was achieved. Both the kinetic and mass transfer aspects were also evaluated and the results implied that the biphasic system was under mass transfer control. In addition, supported aqueous phase (SAP) catalysts of the RhCl(TPPTS)3 solution supported on fine silica were prepared and tested for the selective hydrogenation of cinnamaldehyde under similar conditions for comparison and they gave a very high selectivity to hydrocinnamaldehyde which was an advantage, although their activity was lower than the biphasic catalysts. The mass transfer characteristic in SAP catalysis was also evaluated using a first-order film model. (C) 2004 Elsevier Ltd. All rights reserved.