The effect of wastewater particles on catalytic ozonation during the treatment of petrochemical secondary effluent (PSE) was investigated. The dissolved organic carbon (DOC) removal results indicated that wastewater particles could significantly affect the catalytic oxidation reaction and increase ozone (O-3) consumption. When the suspended solids (SS) was filtered by the 0.45 mu m membrane, the O-3 dosage to DOC removal ratio was 1.99 mg(O3) mg(DOC)(-1), while it increased to 2.99 mg(O3) mg(DOC)-1 when the SS concentration was 50 +/- 1.57 mg L-1. Wastewater particles can react with O-3, thus reducing the O-3 and hydroxyl radicals (% OH) available for DOC removal. When the SS concentration was 30 +/- 0.96 mg L-1, O-3 dosages lower than 10 mg L-1 did not induce a change in the size and surface characteristics of the particles. Following the increase in the O-3 dosage to 36 mg L-1, extracellular polymeric substances (EPS) reacted with O-3/(center dot) OH with different characteristics. Dissolved organic matter (DOM) had the highest depletion reaction rate, followed by the loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS). The average reaction rates of the three constituents were 1.44, 0.212, and 0.079 mg L-1 min(-1), respectively. In addition, the results showed that eliminating wastewater particles would be beneficial for the removal of organic micro-pollutants (OMPs) by catalytic ozonation.