Slug flow, one of the ordered multi-phase flow patterns in a small channel, has the advantage of the enhancement of mixing in each phase and mass transfer between two phases due to the internal circulation flow. To form stable slug flow, the throughput of order of mu L min(-1) has been employed. To use slug flow in industrial scale, however, controlled and high throughput mass transfer and an index for design of a channel with slug flow are required. To address this requirement, we examined the influences of operating conditions such as the channel size and the flow rate on flow patterns and the mass transfer rate. Based on the experimental results, we derived an index to represent the mass transfer rate in slug flow. For further improvement of mass transfer in slug flow, we also verified that the channel including a sudden expansion improves the mass transfer efficiency. (C) 2010 Elsevier B.V. All rights reserved.