Two-phase flow hydrodynamics in vertical capillaries of circular and square cross sections were experimentally studied, using air as the gas phase and water, ethanol, or an oil mixture as the liquid phase. The capillary hydraulic diameters ranged from 0.9 mm to 3 mm, with the superficial gas and liquid velocities covering a span of 0.008-1 m/s, which is typical of that obtained in monolith reactors. Using a high-speed video camera, four distinct flow regimes were observed within the range at which experiments were conducted: bubbly, slug-bubbly, Taylor, and churn flows. Annular flow was observed at excessively high gas and low liquid flow rates, well beyond those of interest to this study. Based on the definition of a two-class flow regime, the combination of two parameterssthe slip ratio (S) and the ratio of the superficial gas velocity to two-phase superficial velocity (UG/UTP)-was observed to be suitable for determining the transition from homogeneous flow to nonhomogeneous flow. The influence of capillary geometry, capillary hydraulic diameter, and fluid properties on bubble rise velocity (Vb) were investigated and determined to be of little significance. Furthermore, a new and simplified correlation for predicting Vb and, by implication, the gas holdup (∈G) was proposed. Liquid slug lengths were also experimentally studied, using a correlation that was developed to estimate them. Pressure drop experiments were also performed, and the peculiar phenomenon of negative frictional pressure drops was observed at very low liquid velocities. By defining a new dimensionless quantitysthe pressure factor, FE-a flow-regime-dependent method for estimating the total pressure drop in two-phase vertical capillary flows was developed.