Visual observations of two-phase regimes for R134a and R245fa flowing in 0.509 mm and 0.790 mm horizontal tubes are documented and compared to the predictions of the analytical flow regime models available in the literature. Annular flow was found to dominate the behavior of these two miniature channels, with a significant Slug flow regime at intermediate qualities. Despite the horizontal orientation of these tubes, there were no observations of Stratified flow and a very limited region of Bubble flow. A comparison of the more than 2200 flow regime observations to the predictions of the Taitel-Dukler flow regime methodology revealed that 67% of the empirically observed flow pattern data were correctly identified. The Ullmann-Brauner model, based on an air-water database, correctly predicted the appropriate flow regime for 81% of the reported data. Proposed modifications in the Bubble-to-Slug and Slug-to-Annular transition criteria, respectively, were shown to provide a modest further improvement in the overall predictability to 90% of the observed data for the two refrigerants studied. (C) 2010 Elsevier Ltd. All rights reserved.