The effect of solvent composition (water, 2-propanol and mixtures of the two) on the three phase, heterogeneously catalysed, two-step hydrogenation of 2-butyne-1,4-diol on Pd/alumina was investigated in a stirred reactor and for the first time, bubble sizes were measured in situ simultaneously with reaction rates. The first step forming the alkene is slow and gas-liquid mass transfer limitation is unlikely whilst the second step to alkane is much faster and may be mass transfer dominated. Overall, reaction rates displayed a complex relationship with solvent composition. The smallest bubbles where observed with 5%v/v 2-propanol in water and under gas-liquid mass transfer limited conditions (energy dissipation rates < 4 W/kg), the overall reaction closely follows the ratio of mean bubble diameter to hydrogen solubility, both of which are a function of solvent. Overall, the solvent 5%v/v 2-propanol/water gave faster reactions combined with pod selectivity compared to the others and is therefore recommended. (c) 2007 Elsevier Ltd. All rights reserved.