Most of the studies on MED systems have dealt mainly with the design problem where the number of effects, top brine temperature and seawater temperature are specified and the heat transfer areas and specific heat consumption are calculated. The other problem which did not seem to have received equal attention is the operating problem where it is required to predict the performance of an evaporator, whether it is already existing or has been designed, at different loads. This is particularly important when the evaporator is not running at full load due to variations in the thermal energy supplied to it as in the case of a solar energy source. The main objective of this study is to develop a simplified program to simulate part-load performance of small vertically-stacked MED plants of the HTTF type using hot water as the thermal energy source and to validate this program using experimental data obtained from an existing plant in operation. The thermal analysis is based on the usual assumption of equal inter-effect temperature difference, average latent heat of vaporization and average specific heat. Comparison between measured and calculated values of distillate production and specific heat consumption for different operating conditions shows a prediction error ranging from -3.4% to +4.5% for the distillate production and ranging from -7.4% to +8.4% for the specific heat consumption. Considering the simplified nature of this program as well as the inaccuracies in the measuring instruments, such prediction error can be considered acceptable.