An experimental rig was constructed that enabled vertical co-current two-phase flow to be studied in a 0.026 m i tube. For vertical air-water flow, pressure loss fluctuation characteristics were useful in helping to identify phase patterns present and ensuring consistent operational conditions. Details are presented on the semi-annular flow regime which formed between churn and annular flows.Liquid hold-up was measured using the quick closing valve technique. Measurements showed the liquid hold-up to be flow regime dependent in agreement with published work. Prediction was achieved by the Nicklin(46) and Bonnecaze(19) relations for intermittent flow and the Spedding-Chen relation(17) for annular type flows.