in the south of Algeria, to supply sufficient fresh water for the population, desalination is necessary because water resources (underground and geothermal) are brackish. This paper presents the theoretical study and the results of experiments carried out with a capillary film multi-effect distiller installed in the south of Algeria (in a village near Touggourt, where the temperature of the groundwater is about 65 degrees C at the source). The name of this device is DIFICAP (DIstiller with a Film in CAPillary motion). The aim of our study is to improve the efficiency of this distiller. The research and development of this desalination process is carried out under the following aspects: modelisation of heat and mass transfer, experimentation under direct solar radiation in South Algeria and technical development to aim to optimize the efficiency of this distiller. The theoretical and experimental results show that the efficiency of this distiller increases when the temperature of the brackish water, the intensity of the solar radiation and the number of stages increase.