The present study investigates the effect of tube diameter and shape on flow regime transitions for two-phase flow in tubes with small hydraulic diameters. Flow patterns for co-current flow of air-water mixtures in horizontal round and rectangular tubes are determined by high-speed video analysis to develop flow regime maps and the transitions between these flow regimes. Gas and liquid superficial velocities range from 0.1 to 100 m s(-1), and 0.01 to 10.0 m s(-1), respectively. Bubble, dispersed, elongated bubble, slug, stratified, wavy, annular-wavy, and annular flow patterns are observed. The effect of tube diameter and shape on the flow patterns for hydraulic diameters ranging from 5.5 to 1.3 mm is documented and compared with the literature.