A multiple-effect solar distiller is more effective than a single-effect unit because it uses the available energy more than once. Considerable improvement in productivity may also be obtained if the water vapor in the last-effect unit is carried away directly by the flowing air. Moreover, since an upward-type solar distiller may create a free-convection effect, it is more efficient than a downward-type unit. The energy balances for upward-type, double-effect solar distillers, with air flow through the second-effect unit, are given in this paper. The air humidity at the condensing surfaces, as well as in the outlet air, was assumed to differ from the saturation value by a factor defined to be a ''modified factor''. The effects of various conditions on these modified factors have been investigated experimentally. The use of a computer may facilitate the determination of the theoretical performance under various operating and design conditions.