Action spectra for simulated sunlight were measured in clear water for two viruses: PRD1, a double-stranded DNA bacteriophage, and MS2, a single-stranded RNA bacteriophage. Viruses were diluted into phosphate buffered saline (20 mM PBS, pH 7.5) and exposed for 22 h to simulated sunlight either directly or through one of six glass filters with 50% cutoff wavelengths ranging from 280 to 350 nm. Virus survival was measured using the double agar layer plaque method. Both UVA (320-400 nm) and UVB (280-320 nm) light were found to contribute to PRD1 inactivation, while only UVB inactivated MS2. A computational model was developed for interpreting these action spectra with 3-nm resolution. Using these methods, we provide detailed estimates of the sensitivity of MS2 and PRD1 to photoinactivation from 285 to 345 nm. The resulting sensitivity coefficients can be combined with solar spectra to estimate inactivation rates in clear water under different sunlight conditions. This approach will be useful for modeling the inactivation of viruses and other microorganisms in sunlit natural and engineered systems.