Substrate toxicity can impose operability challenges in the biological treatment of xenobiotic compounds. These can arise from transients in the feed to conventional continuous processes, as well as in more challenging systems, sequencing batch reactors (SBRs), whose operation is always inherently dynamic. Via the use of the classic Haldane model for microbial toxicity, and unsteady-state material balance equations for SBR operation, we have analysed the behaviour of an SBR reactor operating over multiple cycles and have shown that both high performance and low performance can be obtained depending on the feed substrate concentration and the selected SBR exchange ratio. That is, both the intrinsic microbial kinetics as well as the selection of process operating conditions can lead either to high performance (high removal efficiency) or low performance (low removal efficiency) operation. Using this approach, we have also discussed the performance for the SBR treatment of 2 "real" substrates possessing widely different kinetic parameters, showing the impact of these parameters, as well as process operating conditions, on the operability of SBR biotreatment systems handling xenobiotic compounds. This could be of significant value to practitioners wishing to select high performance operating regimes for the treatment of a specific xenobiotic compound. To our knowledge, this is the first systematic study of the impact of substrate toxicity on SBR operability, and is also a first step in modeling the impact of substrate detoxification, via the use of discontinuous Two-Phase Partitioning Bioreactors, on the dynamic performance of xenobiotic biotreatment processes. (C) 2013 Elsevier B.V. All rights reserved.