In this research, the critical heat flux (CHF) due to flooding limit of thermosyphon heat pipe using triethylene glycol (TEG)-water mixture has been investigated. From the experiment it is found that, use of TEG-water mixture can extend the heat transport limitation compared with pure water and higher heat transfer is obtained compared with pure TEG at high temperature applications. Moreover it is found that ESDU equation is appropriate to predict the CHF of the thermosyphon in case of TEG-water mixture. For thermosyphon air preheater at high temperature applications, it is found that with selected mixture content of TEG-water in each row of the thermosyphon the performance of the system could be increased approximately 30-80% compared with pure TEG for parallel flow and 60-115% for counter flow configurations. The performances also increase approximately 80-160% for parallel flow and 140-220% for counter flow compared with those of pure dowtherm A which is the common working fluid at high temperature applications. © 2002 Elsevier Science Ltd. All rights reserved.