The need for meteorological parameters is essential in the design and study of solar energy conservation devices. Other uses of such information include agricultural studies, meteorological forecasting, environment, and energy conversion. In this study, the ratio of monthly average hourly diffuse radiation to monthly average hourly global radiation (I-d/I) was correlated with the monthly average hourly clearness index (k(T)) in the form of the polynomial relationships for the city of Izmir in the western part of Turkey. Experimental data were measured in the Solar-Meteorological Station of Solar Energy Institute in Ege University over a 5 year period from 1994 to 1998. The values of the monthly average-daily hours of bright sunshine (S) varied between 3.54 and 12.28. The values of the monthly average-daily clearness index (K-T) ranged from 0.41 to 0.66, averaged for the same period. The frequency distribution of the clearness index was also given. It can be concluded that the present models estimate the values of I-d for Izmir, and possibly elsewhere with similar climatic conditions, reasonably well if the values of I are known or can be predicted.