Ultraviolet solar irradiance (290--385 nm) was analyzed in Valencia and Cordoba, Spain, during 1996--1997. Its relation to the instantaneous atmospheric clearness index kt, the relative optical mass mr, the time of the year, and the total or global irradiance on a horizontal surface was established. The maximum values of UV irradiance for each optical mass exponentially diminished with the optical mass, thus for Cordoba and Valencia, Guv=62.23 mr-1.34 and Guv=52.84 mr-1.30, respectively. The UV irradiance in Cordoba represented 4.2% of the total irradiance while in Valencia this percentage was 4.9%. In the case of cloudy days (kt<0.4), these percentages ranged between 5.6% and 6.8% in Valencia and between 5.1% and 5.5% in Cordoba. ¿ 2000 American Geophysical Union