Solar irradiation and sunshine duration are intimately related phenomena and a model involving the relationship between them can best be derived by methods that explicitly take vagueness into account. In this paper, the theory of fuzzy sets, especially fuzzy modeling, is employed to represent solar irradiation and sunshine duration relations as a set of fuzzy rules. A fuzzy logic algorithm has been devised for estimating the solar irradiation from sunshine duration measurements. The main advantage of fuzzy models is their ability to describe the knowledge in a descriptive human-like manner in the form of simple rules using linguistic variables only. In this manner the classical Angstrom or any other type of regression equations can be replaced by a set of fuzzy rule bases. The fuzzy approach is applied for three sites with monthly averages of daily irradiances in the western part of Turkey. (C) 1998 Elsevier Science Ltd. All rights reserved.