Nowadays, the demand for ultrapure water (UPW) in the semiconductor industry is increasing with the booming developments in semiconductor technology. Vacuum-ultraviolet (VUV) lamp emits 185nm, 254nm wavelengths light simultaneously and is used to remove dissolved organic carbon. At 185nm, water decomposes to various radicals which generate hydrogen peroxide (H₂O₂). When H₂O₂ remains in UPW, the semiconductor wafer can be oxidized which results to low quality product. The objective of this study is to elucidate the mechanism of H₂O₂ formation through the VUV process in the presence of low molecular weight organic substances. Methanol, isopropyl alcohol, nitromethane, and acetonitrile were selected as representative low molecular weight compounds due to their detection in the final effluent of UPW production. The findings confirm a direct correlation between the concentration of organic matter and the production of H₂O₂, with higher concentrations leading to increased H₂O₂ levels. Furthermore, a decrease in pH was associated with elevated H₂O₂ production indicating that H^• is important factor in H₂O₂ formation. However, across all pH ranges, H₂O₂ was not generated in the absence of oxygen, underscoring the crucial role of dissolved oxygen in H₂O₂ formation. Computational fluid dynamics (CFD) simulations validated these findings, indicating that O₂^•- and H^• are the primary radicals responsible for H₂O₂ formation in the VUV oxidation process. Moreover, the simulation showed that higher concentrations of organic matter correlated with increased H₂O₂ generation as a result of the R radical chain reaction with hydroxyl radicals. These analytical data may help understand dominant radical pathway inside an annular reactor.