This investigation demonstrates that in situ respirometry can be an effective tool to manage the removal of an inhibitory substrate in a sequencing batch reactor (SBR). Data collected enabled the determination of an optimum operating cycle for the effective treatment of a synthetic wastewater containing up to 1300 mg/l phenol as the sole carbon source. Oxygen uptake rates were monitored in situ at various stages of a bench-scale sequencing batch reactor. Respirometry was used as an indicator of microbial activity and substrate utilisation. Although the profile of the Oxygen Uptake Rate (OUR) followed closely that of phenol substrate removal, any correlation between the OUR and soluble residual substrate COD was specific to that system. A high OUR Value corresponded to the feed period; at the end of the cycle, when the substrate was depleted, the OUR Value was low. A plot of OUR values provided a good indication of the biological activity in the reactior. It was possible to select an optimum operating cycle using the oxygen uptake rates as an indicator for the removal of phenol. The initial operating cycle was 24 h, which included phases for filling, reaction, quiescence and decanting. From oxygen uptake rates and corresponding soluble COD values of the remaining substrate, data showed that the 4 h operating cycle was able to achieve similar substrate removal efficiencies. At a SBR loading rate of 3.12 kg phenol m(-3)d(-1), removal of 97% COD was achieved at the end of the cycle. The reactor was operated at a SRT of 4-27 days with biomass concentrations ranging from 3500-3900 mg/l. (C) 1999 Elsevier Science Ltd. All rights reserved.