This paper presents the thermal performance of a membrane distillation (MD) solar desalination unit located at the Mechanical Engineering Department site, Alexandria University, Alexandria, Egypt. The unit is designed and installed as a part of a partially funded European Commission (EC) project named "PV and thermally-driven small-scale, stand-alone desalination system with very low maintenance needs (SMADES)". The basic innovative MD principle and module is highlighted. The Alexandria MD unit is described and the unit performance is presented for the 6 months of its operation period. The unit performance covers the transient changes in the unit productivity, unit feed water, brine and product water temperatures and conductivities, unit salt rejection, solar collector and MD process efficiencies. The unit performance, in clear and cloudy days, for typical summer and winter months, is presented. For a sunny day of, say, 7.25 kWh/d the results indicate that the unit produces about 11.21/d for every m(2) of the collector area. The relatively high productivity, above that of the conventional solar still, is due to the partial recovery of the condensation energy. The overall unit productivity has been correlated against the solar irradiation as: Daily production (1/m(2).A) = 1.666 q Daily radiation (kWh/m(2).W) - 1.67.