The aim of this study was to estimate the BOD5 and COD removal efficiency and biomass yield coefficient in sequencing batch reactors (SBR) treating landfill leachate. Experiments were carried out in four SBRs at HRT of 12, 6, 3 and 2 d. Two series were performed. In series 1, the reactors were operated in a 24 It cycle mode (anoxic 3 h, aeration 18 h, settling 2.75 h, and discharge 0.25 h). In series 2, however, the anoxic phase was eliminated. In both series the BOD5 removal efficiency was almost identical - over 98%. On shortening HRT from 12 to 2 d, COD removal efficiency decreased from 83.1% to 76.7% (series 1). In series 2, efficiency ranged from 79.6% to 75.7%. In the reactors working with the anoxic phase the observed biomass yield coefficient (Yobs) was nearly constant (0.55-0.6 mg VSS/mg COD). Upon elimination of the anoxic phase, the Y-obs was observed to decrease from 0.32 mg VSS/mg COD (HRT 2 d) to 0.04 mg VSS/mg COD (HRT 12 d). (c) 2006 Elsevier Ltd. All rights reserved.