Monolith reactors offer several advantages over traditional random fixed beds or slurry reactors, such as better mass transfer characteristics, higher volumetric productivity for a smaller amount of catalyst, elimination of filtration step and lower pressure drop. However, achieving an even gas-liquid distribution across the monolith is difficult. The cocurrent downflow contactor (CDC) reactor presents an elegant solution to this problem by establishing a stable two-phase dispersion. When comparing the CDC reactor with traditional stirred tank and trickle bed reactors for the hydrogenation of 2-butyne-1,4-diol, greater selectivity towards the alkene intermediate is observed in the CDC for a range of solvents. This is partly due to a high dispersion (67%) of Pd on the washcoat support of the monolith and also to the optimisation of the surface species concentrations as a result of greater mass transfer of hydrogen to the catalyst surface compared with alternative reactor designs. Furthermore, when considering the effect of scale-out on the monolith by comparing a single capillary with 1256 and 5026 channel monoliths, initial reaction rates and selectivity were maintained. This supports the use of the CDC as a suitable method for dispersing gas-liquid mixtures through a monolith. (C) 2007 Elsevier B.V. All rights reserved.