This paper presents an experimental study on the impact of channel shape to gas-liquid catalytic reactions inside small reaction channels (1-2mm size). The channel shape is a unique design parameter offered by a structured catalyst/reactor. Comparative reaction testing of square channels vs. circular channels is conducted under various flow conditions in a gas/liquid co-current down flow mode. Hydrogenation of olefins and toluene saturation over the Ni/gamma-alumina monolith catalyst module are used as model reactions. The gas/liquid feed is directly delivered into one, single channel in the monolith catalyst module in order to eliminate any gas/liquid distribution problem. It is found that for the 2-mm channel size, the circular shape gives a catalytic conversion activity two to five times higher than the square-shaped channel. The dramatic activity enhancement is believed resulting from better gas/liquid/catalyst contacting in the circular channel than in the square channel. The degree of activity enhancement due to the channel shape decreases as the channel size is reduced from 2 to 1 mm, because the catalyst utilization is increased with decreasing the channel size. This finding suggests that catalyst utilization in the three-phase reactor can be largely affected by the non-uniformity of flow structure at particle size scale. (C) 2004 Elsevier Ltd. All rights reserved.