A new criterion for the effectiveness of the interfacial area in gas-liquid microreactors is proposed. The criterion should be met in order to avoid saturation of the liquid film covering the channel walls in the case of physical absorption, or liquid reagent exhaustion in this film for absorption with chemical reaction. The criterion takes the form: LD/u delta(2) << 1. For slow reaction catalysed by a solid catalyst deposited on the channel walls the reverse criterion should be met. (c) 2007 Elsevier Ltd. All rights reserved.