The transformation of the artificial sweetener acesulfame by direct photolysis was investigated at various pH values, in different water types and at various concentration levels. Main photodegradation products of acesulfame were elucidated and analyzed both in laboratory experiments and in a full-scale waterworks using UV treatment for disinfection purposes. The degradation of acesulfame was found to be independent of the pH (range 5-11) and followed pseudo first order kinetics in a concentration range between 1 μg∙L(-1) and 10 mg∙L(-1). Calculated rate constants were in the range between 5.4·10(-3)s(-1) and 7.4·10(-3)s(-1). The main photodegradation products of acesulfame were separated by ion exchange chromatography and high performance liquid chromatography and were identified as hydroxylated acesulfame and iso-acesulfame by high resolution mass spectrometry and fragmentation experiments. In the case of iso-acesulfame an intramolecular rearrangement is assumed as the transformation product has a higher polarity and different product ions after MS fragmentation compared to acesulfame. Minor transformation products were identified as amidosulfonic acid and sulfate by comparison with analytical standards. The transformation pathway was found to be transferable to drinking water production as the identified transformation products were also detected to a similar extent in fortified tap water. In a Swiss full-scale waterworks acesulfame concentrations were reduced by approximately 30% and one of the main UV transformation products could be qualitatively detected.