The Angstrom turbidity coefficient, the Linke turbidity factor, and the Unsworth-Monteith coefficient have been determined and analysed based on measurements of normal direct irradiance and global horizontal irradiance taken in Valencia, Spain, between January 1990 and December 1996. The data, which were acquired automatically and continuously, have been filtered to select only those values corresponding to clear sky conditions. To determine the Angstrom turbidity coefficient the method proposed by Louche et al. has been used whilst the expression for delta(CDA) proposed by Kasten was used to obtain the Linke turbidity coefficient. The Angstrom turbidity coefficient showed a minimum in winter with values between 0.06 and 0.12. It tended to increase in the spring and reach a maximum between 0.22 and 0.29 in summer before falling again in the autumn. The daily and annual evolution of all three coefficients were very similar with correlation coefficients between pairs of them close to 1.