BACKGROUND: In the framework of biological processes used for waste gas treatment, the impact of the inoculum size on the start-up performance needs to be better evaluated. Moreover, only a few studies have investigated the behaviour of elimination capacity and biomass viability in a two-phase partitioning bioreactor (TPPB) used for waste gas treatment. Lastly, the impact of ethanol as a co-substrate remains misunderstood. RESULTS: Firstly, no benefit of inoculation with a high cellular density (> 1.5 g L-1) was observed in terms of start-up performance. Secondly, the TPPB was monitored for 38 days to characterise its behaviour under several operational conditions. The removal efficiency remained above 63% for an inlet concentration of 7 g isopropylbenzene (IPB) m(-3) and at some time points reached 92% during an intermittent loading phase (10 h day(-1)), corresponding to a mean elimination capacity of 4 x 10(-3) g L-1 min(-1) (240 g m(-3) h(-1)) for a mean IPB inlet load of 6.19 x 10(-3) g L-1 min(-1) (390 g m(-3) h(-1)). Under continuous IPB loading, the performance of the TPPB declined, but the period of biomass acclimatisation to this operational condition was shorter than 5 days. The biomass grew to approximately 10 g L-1 but the cellular viability changed greatly during the experiment, suggesting an endorespiration phenomenon in the bioreactor. It was also shown that simultaneous degradation of IPB and ethanol occurred, suggesting that ethanol improves the biodegradation process without causing oxygen depletion. CONCLUSION: A water/silicone oil TPPB with ethanol as co-substrate allowed the removal of a high inlet load of IPB during an experiment lasting 38 days. (c) 2008 Society of Chemical Industry.