Laboratory tests underline the improvement which ozonation brings about in the removal of organic micropollutants in surface water with activated carbon filtration. Optimal ozone dosages correspond to the stabilization of the asymptotical diminution of UV absorption. HPLC analysis shows that, though ozone sometimes degrades organic compounds into more polar and less adsorbable products, improvement in GAC filtration is attained through increased bacterial activity, as revealed by measurements of ATP, of dehydrogenase and of oxygen consumed.