The effects of target compounds, co-existing substances, and their concentrations on the performances of ozone/vacuum ultraviolet process (O3/VUV) were investigated. The decomposition rates of malonate, acetate and oxalate ions were different one another depending upon the structure of the target compounds including their decomposition by-products and their concentrations. The study on the effects of co-existing substances (i. e., bicarbonate ion, nitrate ion and humic acid) revealed that humic acid is a stronger scavenger than bicarbonate or nitrate ions, and that nitrite formation during the simple VUV radiation (i. e., without ozonation) of a solution containing nitrate ion is depressed with ozonation with and without organic compounds.