The hydrogenation of para-toluidine was studied over a series of rhodium/Silica catalysts. The reaction exhibits an antipathetic particle size effect, suggesting that plane face surface atoms, such as C-9(3) sites, are the active site for ring hydrogenation. The reaction is zero-order in p-toluidine and first-order in hydrogen, with the mechanism of formation of the cis and trans isomers likely through surface immine and enamine intermediates. The activation energy for the trans isomer (62 +/- 4 kJ mol(-1)) was systematically greater than that of the cis isomer (51 +/- 6 kJ mol(-1)). The support was also shown to play a part, adsorbing trans-4-MCYA in the early stages of the reaction, resulting in a varying cis:trans ratio. The effect of pore size was also studied; a dramatic decrease in activity was observed at pore diameters < 64 angstrom. (c) 2006 Elsevier Inc. All rights reserved.