Capillary zone electrophoresis (CZE) with on-line UV-detection was used for the determination of arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine and arsenocholine. The method is simple and rapid (< 10 min) and allows the determination of six different arsenic species without sample pretreatment. Several instrumental parameters were studied to obtain the best performance (pH of buffer, injection mode, injection time, applied voltage). To determine the arsenic compounds, the instrument was used with a negative potential applied to the injection side of the capillary so that the anions can migrate towards the anode because of their own mobility and charge. The capillary wall was coated with an electro-osmotic how modifier which reversed the electro-osmotic flow and thus increased also the overall migration of the anions towards the anode. The influence of high concentrations of matrix components such as NaCl, KNO3 and NaNO3, as well as the presence of acids such as HNO3 and HCl was studied. CZE was used for the determination of the oxidation state of arsenic in percolate waters and in the leachate of solidified arsenic containing waste. The lowest detectable concentration was about 100 mu g/l. A comparison with the results obtained with hydride generation coupled to ICP-MS was made.