The combination of low-dose ozone with ultraviolet (UV) irradiation should be an option to give benefit to disinfection and reduce drawbacks of UV and ozone disinfection. However, less is known about the disinfection performance of UV and ozone (UV/ozone) coexposure and sequential UV-followed-by-ozone (UV-ozone) and ozone-followed-by-UV (ozone-UV) exposures. In this study, inactivation of E. coli and bacteriophage MS2 by UV, ozone, UV/ozone coexposure, and sequential UV-ozone and ozone-UV exposures was investigated and compared. Synergistic effects of 0.5–0.9 log kill on E. coli inactivation, including increases in the rate and efficiency, were observed after the UV/ozone coexposure at ozone concentrations as low as 0.05 mg·L^-1 in ultrapure water. The coexposure with 0.02-mg·L^-1 ozone did not enhance the inactivation but repressed E. coli photoreactivation. Little enhancement on E. coli inactivation was found after the sequential UV-ozone or ozone-UV exposures. The synergistic effect on MS2 inactivation was less significant after the UV/ozone coexposure, and more significant after the sequential ozone-UV and UV-ozone exposures, which was 0.2 log kill for the former and 0.8 log kill for the latter two processes, at ozone dose of 0.1 mg·L^-1 and UV dose of 8.55 mJ·cm^-2 in ultrapure water. The synergistic effects on disinfection were also observed in tap water. These results show that the combination of UV and low-dose ozone is a promising technology for securing microbiological quality of water.