Removal of odorous aldehydes by UV/H2O2 was compared to removal of geosmin and 2-MIB by the same process. Odour transformation was investigated by sensory test and by-products were monitored by a carbonyl derivatization method. Heptadienal, decadienal and nonadienal were removed faster than geosmin and 2-MIB. The primary mechanism was the direct UV photolysis in the UV/H2O2 process. In sensory tests, new odours such as chalky or sweet odours were produced while the initial odour intensity of fishy/grassy-smelling aldehydes was reduced with increasing exposure time to UV/H2O2. New carbonyl compounds were detected from the UV photolysis of nonadienal and were not removed by further UV irradiation, which was thought to be related to production of new odours. Results indicate that new types of odour were produced from the oxidation of odorous aldehydes, and consequently, sensory tests coupled with chemical analysis should be considered in designing an oxidation process to control recalcitrant odorants.