The relevance of the research is determined by the need to solve the problem of toxic substances, including arsenic compounds, entry into water bodies, by searching for and developing effective photochemical methods for cleaning and neutralizing arsenic-containing solutions using environmentally friendly oxidants and modern mercury-free sources of UV radiation. The main aim of the research is to study the possibilities of using excilamps for neutralizing As-containing waters and to establish the regularities of photochemical activation of arsenic oxidation by hydrogen peroxide using a mercury-free source of quasi-monochromatic radiation from the KrCl excilamp. Objects: model aqueous solutions of arsenic (III) with a concentration of 2,7 μM (~0,2 mg/L), prepared on distilled water (UEP=2 μS / cm, pH 5,8) and tap water. Methods: spectrophotometric determination of arsenic (III) ions; pH control by potentiometric method, shooting of electronic absorption spectra of solutions of reacting substances; determination of absorbed emission power of the excilamp by the method of chemical actinometry with atrazine; evaluation of doses of UV radiation. Results. The authors have established the principal possibility of using the mercury-free source of quasi-monochromatic radiation of the KrCl excilamp (222 nm) for photochemical oxidation of As (III) by the oxidation system {UV/H₂O₂}. They studied the main regularities of photochemical oxidation of arsenic (III) using the mercury-free UV source KrCl-excilamp (222 nm) and carried out the comparative studies of the efficiency of direct photolysis and combined oxidation of arsenic (III) in aqueous solution with ultraviolet radiation of the KrCl excilamp in the presence of hydrogen peroxide. According to the efficiency of As (III) oxidation the considered oxidation systems may be arranged as {UV/H₂O₂}>{H₂O₂}>{UV}. At the combined treatment of {UV/H₂O₂} the significant intensification of oxidation is observed, after 10 minutes the residual arsenic concentration in the solution is below the MPC (0,01 mg/l). The paper considers as well the effect of hydrocarbonate ions, being the most characteristic anions of natural «water matrices», on the oxidation rate of As (III). The authors proposed the ion-radical mechanism of arsenic photochemical oxidation in a combined oxidation system {UV/H₂O₂} using the UV radiation of the KrCl excilamp.