In this study, the effect of tube diameter and type of distributor on the flow patterns during cocurrent, downward, gas - liquid flow has been investigated. The superficial velocities of the liquid phase (water) and the gas phase (air) were varied in the range of 0.01 - 0.25 m/s and 0.01 - 0.5 m/s, respectively. For this purpose, capillary tubes of two diameters (1 and 1.89 mm) and two different distributors (conical and Y) were used. Three distinct flow regimes were observed: bubble flow, slug (or Taylor) flow, and liquid-ring flow. Flow transitions were affected by the type of distributor and the size of the capillary. The gas and liquid slug lengths were found to depend on the type of distributor, the capillary diameter, and the superficial fluid velocities. These slug lengths were compared with three published correlations, but none of these were found to be satisfactory. To have general applicability, any future correlation should account for the distributor design. For both distributors and capillary sizes, the bubble velocity was higher than the mean velocity (U-m) and varied linearly with U-m.