In this paper, the effect of sonication on the UV disinfection kinetics of primary effluents was investigated. Wastewater samples were collected from local municipal treatment plants and were sonicated with a 20-kHz ultrasound reactor at constant power but varying sonication times. Sonicated samples were irradiated using low-pressure UV light to obtain the UV dose-response curves (DRC). Results showed that sonication improved the UV disinfection of primary effluents by (1) increasing the initial slope of DRC (i.e., k1) and (2) decreasing the tailing level of the UV dose-response curve (i.e., beta). This improvement was confirmed to be caused by the breakage of large particles (> 60 microm) that are known to protect coliforms from UV photons. It also was found that the log reduction of the tailing level of DRC was directly proportional to the log reduction of the number of large particles (> 60 microm) present in the effluent sample. Although the number of large particles was proportional to the coliform count at high UV dosage, the proportionality constant varied from 0.05 to 0.25, depending on the sample.