This study aimed at explaining the differences in the removal efficiencies of 12 polycyclic aromatic hydrocarbons (PAHs, from low to high molecular weight) stated during the ozonation of contaminated sewage sludge. Correlation between PAH removal efficiency at fixed ozone doses and the characteristics of PAHs (molecular weight, number of aromatic rings, number of 5-carbon rings, solubility in water, octanol-water partitioning coefficient) was established through partial least-squares (PLS) regression. PAHs removal efficiency was primarily determined by the solubility of PAHs in water and the number of 5-carbon rings. Considering six ozone doses (from 0.5 to 4 g/L), the impact of added ozone on PAH removal efficiency decreased. PLS analysis suggested that 77% of the ozone efficiency variability was a result of the sludge soluble chemical oxygen demand (SCOD) concentration in sludge. The competition between SCOD and PAHs was confirmed by ozonation experiments in the presence of surfactants. The use of surfactants actually led to higher gas-liquid transfer coefficients and to higher PAH removal efficiencies, which shows that ozone concentration was the limiting factor. (c) 2006 American Institute of Chemical Engineers.