In a pilot scale shell and tube type photocatalytic reactor and a batch type lab scale photocatalytic reactor, the biologically pretreated landfill leachate from Kimpo landfill in Korea characterized to have low biodegradability (BOD/COD = 1.1%) was tested. The role of DO (dissolved oxygen) was important in photocatalysis, thus we monitored the DO at the inlet and outlet points of the pilot scale photocatalytic reactor. DO consumption in a pilot scale photocatalytic reactor was monitored as the index of reaction progress. However, complete mineralization could not be achieved due to the photocatalyst deactivation. After photocatalytic reaction, the residual species in the landfill leachate were identified with GUMS. The residual materials were siloxane compounds, which might be adsorbed on photocatalyst. These adsorbed species would block the light source and the approach of DO to photocatalyst surface. Therefore, oxygen attachment onto the photocatalyst surface is difficult when no more degradation occurs. (C) 2003 Elsevier B.V. All rights reserved.