The energy requirements for the distillation of impure water can be reduced by using either an open-cycle or a closed-cycle heat pump. The purification of typical sea-water, with a concentration of 3Â·4 weight %, is considered using an open-cycle mechanical vapour-compression (OMVC) heat pump and also a closed-cycle mechanical vapour-compression (CMVC) heat pump using an external working fluid. Plots are given showing the variation of the thermodynamic properties of sea-water with temperature and concentration. Plots are also given of the energy loads against temperature-lift for both the OMVC and CMVC systems. At temperatures up to 90Â°C, the work load on the compressor is less for the CMVC than for the OMVC system. The operating characteristics for a number of possible working-fluids for the CMVC system are also considered.