The formation of disinfection by-products (DBPs), including chloroform, dichloroacetic acid (DCAA), trichloroacetic acid (TCAA), and cyanogen chloride (CNCl) after sequential exposure of four organic waters to UV irradiation via either low- or medium-pressure lamps and free chlorine (or preformed monochloramine) under practical conditions was simulated. Statistically significant changes in the DBP formation from chlorination due to the additional UV irradiation are commonly observed under testing conditions, although some of these changes are not practically significant. The impacts from UV exposure were found to be most significant in chloroform formation (up to 40 microg/L) among the four tested DBPs. Organics from rivers were more sensitive to UV alteration than was the organic drawn from soil. This difference could not be explained by the specific UV absorbance (SUVA) values. In most cases, irradiation with the medium-pressure UV lamp gave similar or slightly larger changes in DBP yields, compared with the corresponding trials using the low-pressure lamp. Different application sequences could significantly change the relative quantities of DBPs but no general trend was identified. Case-specific evaluation of the formation of chloroform and CNCl is necessary.