In the present study, a three-factor central composite design (CCD) was implemented to evaluate the major factors (treatment time, initial H2O2 dose and initial diethyl phthalate (DEP) concentration) influencing the H2O2/UV-C treatment of aqueous DEP solution. Significant regression models for total organic carbon (TOC) removal and residual H2O2 concentration were derived using analysis of variance (ANOVA), which were found to be adequate to perform the process variables optimization. According to the applied ANOVA, treatment time (positive effect) and initial H2O2 dose (negative effect after an optimum value) were found to be significant on the H2O2/UV-C process performance. TOC removal efficiencies were enhanced with increasing initial H2O2 dose up to an optimum value, and further increases in H2O2 dose resulted in a decrease in TOC removals due to the hydroxyl radical scavenging effect. When initial H2O2 dose increased from 10 to 30 mM, the TOC removal efficiency improved from 41 to 100% for DEPo of 100 mg/L for treatment time of 50 min. Further increase in initial H2O2 dose to 50 mM decreased the TOC removal efficiency down to 38%. The results of toxicity assessments with Vibrio fischeri on aqueous DEP solutions subjected to H2O2/UV-C treatment revealed complete detoxification at the optimum conditions.