Visualization experiments were carried out for boiling of propane, isobutane and their mixtures on a smooth and four reentrant enhanced tubes. The results shown by the video pictures indicate a strong mixture effect on nucleation and evaporation processes on the enhanced surfaces. The fluid physical properties also influence the bubble behavior and consequently affect the heat transfer performance. The effects of reentrant enhanced surface geometries are complicated and closely related to mixture effects, fluid properties and working conditions. Based on the analysis of the available visualization results, the different heat transfer performance for different enhanced surfaces and especially the difference between pure fluids and mixtures, as shown in an accompanying paper [Chen et al., Int. J. Heat Mass Transfer, in press], are partially explained. (c) 2005 Elsevier Ltd. All rights reserved.