This article presents a computational study of the co-current downward Taylor,flow of gas bubbles in a viscous liquid within a square channel of 1 mm hydraulic diameter. The three-dimensional numerical simulations are performed with an in-house computer code, which is based on the volume-of,fluid method with interface reconstruction. The computed (always axi-symmetric) bubble shapes are validated by experimental flow visualizations for varying capillary number. The evaluation of the numerical results for a series of simulations reveals the dependence of the bubble diameter and the interfacial area per unit volume on the capillary number. Correlations between bubble velocity and total superficial velocity are also provided. The present results are useful to estimate the values of the bubble diameter, the liquid film thickness and the interfacial area per unit volume from given values of the gas and liquid superficial velocities. (C) 2009 American Institute of Chemical Engineers AIChE J,56:1693-1702,2010