For vertical air-water flow, five flow regimes were identified by a combination of visual/ video techniques and pressure loss fluctuation characteristics in an 0.026 m i tube. Checks against existing how regime maps indicated that it was unlikely that a universal map will be derived that will correctly predict phase regimes in vertical flow.Existing theoretical transition equations were shown to have difficulty in handling the effect of diameter on the slug to churn transition. The churn to annular transition proved hard to predict at higher liquid rates. Two new relations are presented that predicted the transitions for the slug to churn and churn to annular regimes.