Genetic algorithm analysis of the monthly average daily irradiation and sunshine duration data of four Turkish locations has been performed for determining the Angstrom equation coefficients. This analysis provides the parameter estimations as new data are recorded and consequently yields the time variability of the parameters. Application of the genetic algorithm procedure is presented in this paper for determining the coefficients and their statistical distributions as well as the relationships between them. Good correlation is obtained (r greater than or equal to 0.93) in all the cases, showing the validity of the Angstrom equation for Turkish locations. Besides, the analysis presents a new way of estimating the Angstrom equation parameters. (C) 2000 Elsevier Science Ltd. All rights reserved.