Firstly, it is seen that the reaction timescale depends measurably on the availability of H2 in the systemat higher pressures it takes less time for the reaction to complete (Fig. ), whereas increasing the reaction volume at the same pressure and flow rate of H2 slows the reaction down (Fig. vs. Fig ). These observations are in accord with the accepted reaction mechanism for hydrogenation over cationic rhodium complexes, in which coordination of the unsaturated substrate to the complex takes place first, followed by the oxidative addition of H2 in the ratedetermining step . Higher H2 pressures and smaller sample volumes lead to higher concentrations of dissolved H2 and accelerate the reaction.