Abstract: Vertical thermosyphon reboilers sufferd seriously from being unstable. The problem can be solved by a newly developed internal thermosyphon re-boiler. Experiments were carried out in both quartz and copper tubes of similar geometrical dimensions. The former allowed better visual observation and the latter gave more quantitative informations. Water, sulfuric acid, tri-ethanolamine, heavy oil, ethanol-water, methanol-ethanol, ethanol-propanoland other mixtures were used as working media. Particles of polypropene, with densities approaching that of water were used as tracer in the water system for the investigation of the flow characteristics. Rate of circularion, flow patterns, rate of heat transfer and fouling were studied. A flow model and a design approach were proposed. Calculated results could be used to predict the characteristics of a two-phase flow and its heat transfer within the reboiler.