Expressions for water and glass temperatures, yield and efficiency of both single and double slope multiwick solar distillation systems in quasi-steady state conditions have been derived. The analysis is based on the basic energy balance for both the systems. A computer model has been developed to predict the performance of the solar stills. Experimental validation of the thermal model has been carried out by using modified heat transfer coefficients. Internal heat transfer coefficients have been evaluated based on both inner and outer glass cover temperatures for typical days namely January 22, and June 19, 2001 in Delhi. A fair agreement has been observed between theoretical and experimental results by using the modified internal heat transfer coefficients based on inner glass cover temperature. (C) 2004 Published by Elsevier Ltd.