There has been considerable doubt as to the manner in which the productivity of solar stills is affected by many of the designs and operating variables. To assist in designing solar stills of improved performance, theoretical equations are derived to describe the complete energy and mass transfer relationships involved in the operation of the basin type of solar still. These are supplemented with data from field operation of a 2,500-sq. ft. still. With these relationships and the aid of a digital computer, the effects of variations of design parameters on the performance of solar stills is predicted. Distiller productivity is correlated with atmospheric temperature, wind velocity, solar radiation, absorptivity and slope of transparent cover, and other variables. Curves showing the magnitude of the effects of design changes on cover temperature, brine temperature, and productivity are presented.