The performance of sequencing-batch reactors (SBRs) for the treatment of three kinds of wastewater (synthetic phenolic wastewater; synthetic wastewater containing polyvinyl alcohol (PVA), and effluent from a coke-plant-wastewater treatment system) was investigated. Under low-strength phenol conditions, the performance of the SBR with unaerated fill was superior to the SBR with aerated fill in which filamentous bacteria developed. However at high influent phenol concentrations, the SBR with unaerated fill accumulated phenol during the fill period to concentrations inhibitory to microorganisms. Fill mode had no significant influence on the performance of SBRs treating PVA-containing wastewater For the SBRs treating the effluent from a coke-plant-wastewater treatment system, fill mode had no significant impact on the performance of SBRs with respect to organic carbon removal and sludge settleability; but an operating mode which incorporated two anoxic periods, one ahead of the aeration and one after the aeration, was better than other operating modes in terms of nitrogen removal. Copyright (C) 1996 Elsevier Science Ltd.