Coupled heat and mass transfer by natural convection in a rectangular cell of aspect ratio 10 is studied in steady state numerically and experimentally. A wall on which flows a water film is submitted to a constant heat flux while the parallel wall is maintained at a constant temperature. The study aims at the improvement of distillation rate in solar distillation systems. Equations are solved by a finite-volume technique and provide the temperature, concentration and velocity fields in the gas phase for different heat fluxes. The simulation study allowed one to quantify the influences on the mean distillation flowrate of many parameters such as the heat flux, the temperature and the flowrate of the feed water, the temperature of the condensation wall, and the form factor of the cell. Experimental results obtained on a pilot cell concerning desalinated water production confirm the simulation results. For a heat flux equal to 1000 W m(-2), yield equal to 0.7 is commonly obtained.