This paper describes a long term experiment undertaken with six benchscale landfill lysimeters to investigate whether ozone treatment can increase the biodegradability of the leachates arising from the lysimeters, and reduce the gradual build-up of difficult-to-treat ‘hard’ COD, typical of leachates of older landfills. Leachate from all six lysimeters was recirculated through pulverized domestic waste every two weeks. Leachate from four of the lysimeters was ozonated at different doses before recirculation; two lysimeters from the onset of the experiment; and two after a stable methanogenic phase was established. The recirculation of ozonated leachates did not prevent a fully developed methanogenic phase from being established within the lysimeters. Ozone doses of at least 0.06 mg O3per mg COD were found to increase the BOD5/COD ratio, when applied to leachates from lysimeters in a post-methanogenic phase.