Sunlight inactivates waterborne viruses via direct (absorption of sunlight by the virus) and indirect processes (adsorption of sunlight by external chromophores, which subsequently generate reactive species) processes. While the mechanisms underlying these processes are understood, their relative importance remains unclear. This study establishes an experimental framework to determine the kinetic parameters associated with a virus’ susceptibility to solar disinfection, and proposes a model to estimate disinfection rates and to apportion the contributions of different inactivation processes. Quantum yields of direct inactivation were determined for three viruses (MS2, phiX174 and adenovirus), and second-order rate constants associated with indirect inactivation by four reactive species (1O2, OH., CO3.-, and triplet states) were established. PhiX174 exhibited the greatest quantum yield (1.42•10-2), indicating that it is more susceptible to direct inactivation than MS2 (2.9•10-3) or adenovirus (2.5•10-4). Second-order rate constants ranged from 1.7•107 to 7.0•109 M-1s-1 and followed the sequence MS2>adenovirus>phiX174. A predictive model based on these parameters accurately estimated solar disinfection of MS2 and phiX174 in a natural water sample, and approximated that of adenovirus within a factor of three. Inactivation mostly occurred by direct processes, though indirect inactivation by 1O2 also contributed to the disinfection of MS2 and adenovirus.