An experimental investigation of heat transfer to boiling liquids under natural convective flow through a vertical tube thermosiphon reboiler has been carried out to study the effect of heat flux and liquid submergence on circulation rates. Three.test liquids namely: water, methanol and ethylene glycol were used to cover a wide range of boiling points and thermophysical properties. The test section was an electrically heated stainless steel tube 25.56 mm i and 1900 mm length. The uniform heat fluxes in the range of 4.1 to 43 kW/m2 were employed while inlet liquid subcooling varied from 0.5 to 11.6-degrees-C. The liquid submergence levels were maintained around 100, 75, 50 and 30%. Circulation rates for all test liquids have been correlated in terms of dimensionless groups.