Experimental data were obtained for the evaporation of water in a vertical thermosiphon reboiler having 50 tubes of 3 m length and 25.4 mm outer diameter. The effect of step changes in heat load was noted for operation over a pressure range from 0.2 to 1.0 bar. The reboiler behaviour exhibited three 'operational modes', and a definite transitional point that is independent of process pressure was observed between operational modes one and two. The influence of reduced pressures on the vertical thermosiphon reboiler sub-cooled heating length was significant. Overall heat transfer coefficients at 0.2 bar were about 60% of those at 1.0 bar and circulation rates were approximately halved. Two important considerations for the reboiler operation analysis were (a) the possible non-uniform behaviour of the reboiler tubes and (b) the change of phase and processes occurring on the reboiler shell as well as the tube side.