An experimental study of heat transfer to boiling liquids under natural convective flow has been carried out in a single tube vertical thermosiphon reboiler to investigate the effect of heat flux and submergence on circulation rates. The test liquids used were distilled water, various concentrations of propan-2-ol in water and their azeotrope. The test section was an electrically heated stainless steel tube of 25.56 mm i.d., 28.85 mm o.d. and 1900 mm long with twenty-one spot welded copper constantan thermocouples to measure the variations in wall temperature along its axis. The uniform heat fluxes in the range of 3.5-42.4 kW/m(2) were employed while inlet liquid sub-cooling varied from 0.2 to 30.7 degrees C. The liquid submergence levels were maintained in the range 30-100%. The typical experimental data has been graphically presented and discussed. An empirical correlation has been developed from the experimental data of the present study through regression analysis. (c) 2008 The Institution of Chemical Engineers. Published by Elsevier B.V, All rights reserved.