An experimental investigation has been carried out on two-phase flow characteristics in a 100 mum diameter circular tube. Two-phase flow patterns were determined by video recording the flow in the transparent capillary tube made of fused silica, in which de-ionized water and nitrogen gas were injected at superficial velocities of j(G) = 0.1-60 m/s for gas, and j(L) = 0.02-4 m/s for liquid. Time-averaged void fraction and two-phase friction pressure drop data were also obtained and anal; zed. The flow patterns observed were intermittent and semi-annular flows, but a closer study of the liquid film structure revealed gas core flows with a smooth or ring-shaped film and a serpentine-like gas core surrounded by a deformed liquid film. Bubbly and churn flow patterns were not observed. A flow pattern map was developed based on the probability of appearance of each type of flow, and compared to the existing flow pattern maps obtained for similar to1-mm diameter channels. Void fraction remained low even at high gas flow rates, indicating large slip ratios and weak momentum coupling between the phases. The single-phase friction factor and two-phase friction multiplier data were shown to be in good agreement with the conventional correlations. (C) 2002 Elsevier Science Ltd. All rights reserved.