Frictional pressure drops were measured during condensation of refrigerant R134a in small hydraulic diameter (100 < D-h <160 mu m) rectangular (1 < AR < 4) channels using a novel technique to accurately determine frictional pressure gradient at small Delta x. An Annular Flow Factor (AFF) is used to automatically account for the smooth transition between annular and intermittent flow and predict the applicable pressure drop based on a model that accounts for the effects of G, x, T-sat, D-h and AR. The model predicts 95% of the data within +/- 25%. The relative predominance of the two flow regimes predicted by the AFF is in agreement with findings of Coleman and Garimella (2000, 2003). Constant AFF lines may also be used as transition criteria for the application of intermittent or annular flow based models. (C) 2016 Elsevier Ltd. All rights reserved.