Experimental drying studies were made on two types of solar lumber kilns, one an external collector type and the other a semi-greenhouse type. Two charges of green sugar maple lumber (5/4 inches  thick) were tested in an external collector solar kiln at the U.S. Forest Products Laboratory, Madison (43°5'N, 89°23'W), Wisconsin, during the summer of 1982. In the first run detailed drying data were obtained and the energy balance was calculated for each day during the entire drying period. Based on these results, the following empirical model for the overall efficiency of the kiln was obtained: EFF = - .0413 + .0102^*(IMC) - .0000562^*(IMC)² where, EFF = overall efficiency of the kiln IMC = average initial moisture content of the lumber in percent Using  this model, the average daily moisture content loss in percent (MCL) can be calculated as follows: MCL = (100*EFF*SI*ACV)/[R*(62.4*V*SG)*{0.53*(212-Ti)+972}] where, EFF = the value obtained from the first equation ACV = area of the collector in ft² SI = average daily solar insolation in Btu/ft² R = ratio of total solar energy incident on the collector cover to total energy available to the system V = green volume of lumber in ft³ SG = green specific gravity of lumber Ti = average initial temperature inside the kiln in °F A comparison of the actual drying time observed in the second run showed good agreement with the predicted drying time obtained from the above equations. A single charge of 9/8 inches green yellow poplar was dried in a semi-greenhouse kiln at Virginia Polytechnic Institute and State University, Blacksburg (35°09'N, 81°30'W), Virginia, during the fall of 1982. Following the same procedure as for the external collector kiln, an empirical model for the overall efficiency (EFF) of the kiln was obtained as a function of initial moisture content (IMC), EFF = - .0767 + .00988*IMC / M.S.