Abstract After concluding a detailed research on separate pieces of equipment needed for the new desalination process, Phase 1 pilot plant installation for testing and optimizing the whole process was constructed and installed in INRST-Tunisia. The aim of this investigation is to experimentally test the new solar desalination process working with an air multiple-effect humidification–dehumidification technique. A pilot plant unit was designed and constructed to carry out this test investigation. The main parts of the process consist of an air collector field, spray humidifiers and a dehumidifier and heat recovery system. The process flow sheet has undergone several changes: the number of heating–humidifying stages has been lowered from 18 to 15 stages, then to 9 stages, then to 4 stages. The present study concerns Phase 1 installation setup involving two heating–humidifying stages, heat recovery and condensation system and auxiliary equipment (blower, pumps, reservoirs, etc.). Phase 2 installation setup will include the remaining equipment to complete a 4-stage plant. Phase 1 SOLDES pilot plant had been tested during a six-month period (from March to August 2003). The distilled water production had been optimized. Data relative to the operating parameters of each single component of the process (collectors' outlet temperatures, humidifier parameters, recuperated water temperature) are presented. Also, the daily and monthly fresh water production of the pilot plant is obtained. The fresh water production had reached an average value of 355 kg/d during the month of August 2003.