Thermal and productivity measurements and flow visualization experiences were performed on a real scale module of a basin type solar still, whose geometry and thermal conditions could be changed in a controlled way. The convective stage was studied with the aim of acquiring information about the nature of the medium inside it and the influence of different parameters over the productivity. Literature shows a great number of experimental and numerical works dealing with different aspects of the performance of solar stills: thermal losses, vapor losses, salt deposit on the tray, geometry, thermal inertia, etc. Few works are reported that take into account convective phenomena and the fluiddynamic characteristics of the medium inside the still. Most of these works are based on Dunkles and Copper's models of the still that does not take into account the characteristics of the environment. A new physical model based on these experiments is presented.