Human adenoviruses (HAdV) are important pathogens in both industrialized and developing nations. HAdV has been shown to be relatively resistant to monochromatic UVC light. Polychromatic UVC light, in contrast, is a more effective means of disinfection, presumably due to the involvement of viral proteins in the inactivation mechanism. Solar disinfection of HAdV, finally, is only poorly understood. In this paper, the kinetics and mechanism of HAdV inactivation by UVC light and direct and indirect solar disinfection are elucidated. PCR and mass spectrometry were employed to quantify the extent of genome and protein degradation and to localize the affected regions in the HAdV proteins. For this purpose, we used for the first time an approach involving stable isotope labeling by amino acids in cell culture (SILAC) of a human virus. Inactivation by UVC light and the full sunlight spectrum were found to efficiently inactivate HAdV, whereas UVA-visible light only caused inactivation in the presence of external sensitizers (indirect solar disinfection). Genome damage was significant for UVC but was less important for solar disinfection. In contrast, indirect solar disinfection exhibited extensive protein degradation. In particular, the fiber protein and the amino acids responsible for host binding within the fiber protein were shown to degrade. In addition, the central domain of the penton protein was damaged, which may inhibit interactions with the fiber protein and lead to a disruption of the initial stages of infection. Damage to the hexon protein, however, appeared to affect only regions not directly involved in the infectious cycle.