The paper presents flow characterization of condensation based on experiments with R134a in 6.1 mm inner diameter horizontal round tube. The paper presents flow visualizations and liquid film thickness measurements with mass fluxes from 50 to 200 kg m(-2) s(-1) and heat fluxes from 5 to 15 kW m(-2), showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. All the measurements are taken at constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 degrees C. The result of flow visualization reveals that the condensation always starts in the bulk superheated region, which is generally defined as the beginning of condensing superheated region, as annular flow and the flow regime is strongly affected by the mass flux, instead of the heat flux. Based on the flow visualization results and underlying physics in the condensing superheated region, a new diabatic flow regime map is proposed to better represent the physics and predict flow regimes in both the two-phase and the condensing superheated region. The range of prediction of the new flow regime map extends over quality one up to the onset of condensation and the early stages of condensation are strictly in annular flow regime. A film thickness measurement technique for round tube is described as well as results of calibration. The film thickness measurement demonstrates that void fraction drops below one in superheated region, which is not taken into account for most conventional void fraction models. In addition, both the mass flux and heat flux affect the void fraction by altering the onset of condensation, a new void fraction model is proposed to include this mechanism. Moreover, having information of film distribution provides an opportunity to more realistically model the film geometry inside of the tube. (C) 2016 Elsevier Ltd. All rights reserved.