Roxarsone (4-hydroxy-3-nitrophenylarsonic acid) in contact with soil of volcanic origin and chlorine containing water generated a set of organoarsenicals. The transformation products were identified with element-specific (ICP-MS) as well as molecular-specific (ESI-MS) detection after their HPLC separation. The identification of the main transformation products by means of ESI-MS, ESI-MS/MS and ESI-TOF-MS adduce evidence of chlorinated phenylarsonic acids and a phenylarsine oxide derivative which contains arsenic in the trivalent state. Traces of chlorine in water used for sorption experiments are suggested to be responsible for the formation of chlorinated products. After irradiation of a roxarsone solution with visible light, different transformation product so far not identified were detected.