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.