Variations with time in the solar irradiance have been derived from a new empirical model for change in the sensitivity of the solar backscatter ultraviolet (SBUV) instrument aboard Nimbus 7. The SBUV sensitivity change is modeled as the product of two separate exponential decays: one with instrument exposure to the Sun, the other with time. Time constants for these exponentials are calculated from functional fits to the measured UV irradiances for three time intervals, short compared with the instrument lifetime. At nearly all wavelengths the degradation coefficients for exposure dependence (rλ) do not vary significantly from interval to interval, but those for time dependence (sλ) vary significantly with time at the shorter wavelengths. A new instrument sensitivity function is defined to be the product of an exponential decay with exposure with constant rλ and an exponential decay with time with time-varying sλ. The time dependence of sλ is constrained by the values derived for the three fitting intervals. Use of this functional form to correct the measured irradiances for instrument changes yields solar irradiance variations between 175 and 400 nm within 3% of those predicted by the Mg core-to-wing ratio, appropriately scaled with wavelength. This result strongly suggests that the instrument parameters derived during the short fit intervals can be used to improve the representation of instrument behavior during those intervals and can serve as constraints on any function designed to characterize long-term changes in the SBUV instrument. The new characterization yields an estimate for the variation of solar irradiance between maximum and minimum of 5--8% at 205 nm and 1--4% between 210 and 260 nm, consistent with the 8--9 and 3--4% predicted using the Mg index. ¿ American Geophysical Union