The main objective of this study was to assess the combined use of chlorine dioxide(ClO_2)and chlorine(Cl_2) on the speciation and kinetics of disinfection by-product(DBP) formation in swimming pools using synthetic pool waters prepared with a body fluid analog(BFA)and/or fresh natural water. At 1:25(mass ratio) of ClO_2 to Cl_2, there was no significant reduction in the formation of trihalomethanes(THMs) and haloacetic acids(HAAs) for both BFA solution and natural water compared to the application of Cl_2 alone. When the mass ratio of ClO_2 to Cl_2increased to 1:1, substantial decreases in both THMs and HAAs were observed in the natural water, while there was almost no change of DBP formations in the BFA solution. Haloacetonitriles and halonitromethanes levels in both water matrices remained similar. In the presence of bromide, the overall DBP formation increased in both BFA solution and natural water. For the DBP formation kinetics, after 72 hr of contact time,very low formation of THMs and HAAs was observed for the use of ClO_2 only. Compared to Cl_2 control, however, applying the 1:1 mixture of ClO_2/Cl_2 reduced THMs by > 60% and HAAs by > 50%. Chlorite was maintained below 1.0 mg/L, while the formation of chlorate significantly increased over the reaction time. Finally, in a bench-scale indoor pool experiment, applying ClO_2 and Cl_2simultaneously produced less THMs compared to Cl_2 control and kept chlorite at < 0.4 mg/L, while HAAs and chlorate accumulated over 4-week operation period. The main objective of this study was to assess the combined use of chlorine dioxide (ClO_2) and chlorine (Cl_2) on the speciation and kinetics of disinfection by-product (DBP) formation in swimming pools using synthetic pool waters prepared with a body fluid analog (BFA) and / or fresh natural water. At 1:25 (mass ratio) of ClO_2 to Cl_2, there was no significant reduction in the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) for both BFA solution and natural water compared to to the application of Cl_2 alone. When the mass ratio of ClO_2 to Cl_2 increased to 1: 1, substantial decreases in both THMs and HAAs were observed in the natural water, while there was almost no change of DBP formations in the BFA solution. Haloacetonitriles and Halonitromethanes levels in both water matrices are similar. In the presence of bromide, the overall DBP formation increased in both BFA solution and natural water. For the DBP formation kinetics, after 72 hr of contact time, very low formation of T Compared to ClO_2 only. Compared to Cl_2 control, however, applying the 1: 1 mixture of ClO_2 / Cl_2 reduced THMs by> 60% and HAAs by> 50%. Chlorite was maintained below 1.0 mg / L , while the formation of chlorate significantly increased over the reaction time. Finally, in a bench-scale indoor pool experiment, applying ClO_2 and Cl_2simultaneously produced less THMs compared to Cl_2 control and kept chlorite at <0.4 mg / L while HAAs and chlorate over 4-week operation period.