Removal of acenaphthylene from water has been carried out by means of different treatments combining UV radiation, ozone and hydrogen peroxide. Ozonation alone or in conjunction with hydrogen peroxide (10(-3) M) resulted in the highest elimination rates. Thus, conversions as high as 95-100% were obtained in less than 3 min with an ozone dose of 4.1 X 10(-3) mol O-3 h(-1) (flow rate 2 X 10(-2) m(3) h(-1)). Slightly lower efficiencies were experienced when using systems containing UV radiation. By considering the kinetics of the direct photolysis of acenaphthylene and the UV/H2O2 system the photochemical reaction quantum yield phi(A) (4.0 +/- 0.1 x 10(-3) mol/photon) and the rate constant of the reaction of acenaphthylene with the hydroxyl radical k(OH,A) (8.0 +/- 0.5 X 10(9) M-1 s(-1))were calculated. Intermediates identified by GC/MS were in many cases similar regardless of the oxidation treatment used. Most of these by-products constituted oxygenated species of the parent compound (mainly ketones, aldehydes and carboxylic acids) that further degraded to low molecular, harmless end products.