Two-phase pressure drop and overall heat transfer coefficients have been measured in a model horizontal thermosyphon reboiler operating at atmospheric pressure and using water and aqueous solutions of methanol and ethylene glycol. The shell and riser were of glass so that flow patterns could be observed. The aqueous solutions produced much smaller bubbles, but neither the pressure drops nor the overall heat transfer coefficients varied substantially from system to system. Comparison of the pressure drop measurements with available correlations highlighted areas, particularly bends and nozzles, where further study is needed to improve predictions for reboiler design. Overall heat transfer coefficients had the expected order of magnitude (2.5-3.0 kW m(-2) K-1) but did not vary as expected in the solutions. Flow instabilities were noted at low qualities and explored in the water system.