Hydrodynamics of gas-liquid systems has been studied by high-speed camera. The flow patterns of CO2/water-based fluids were investigated in three glass microreactors having different geometries. Four main regimes were observed: bubbly, slug, slug-annular and annular flow. The forces being responsible for the pattern formation were quantified using dimensionless numbers: Bond (Bo), Reynolds (Re), Weber (We) and Capillary (Ca) numbers. The study showed that the interfacial tension and the gas inertia are the main forces that drive the interface. A dimensionless map based on the ratio between the Reynolds and the Capillary numbers of the liquid phase as a function of the Reynolds number of the gas phase was built up for horizontal single channel without heat transfer to rationalise the experimentally observed flow patterns and literature data. Quantitative criteria to predict the slug and the annular flow domain independently on the inlet geometries, reactor material, its cross-section shape and liquid phase properties have been suggested. The criteria can be applied for capillaries with hydraulic diameter from 200 mu m up to 3 mm and is an important predictive tool for the rational design of micro-reactors for gas-liquid reactions. (C) 2010 Elsevier B.V. All rights reserved.