Many researchers have studied the impact of stress/shock imposed due to change in environmental and operational conditions using the same feed substrate type on performances of anaerobic reactors. However, limited reports are available on performance of anaerobic reactors subjected to shock/stress conditions due to sudden change in feed substrate and hence the present work is directed towards this end. Three suspended growth batch reactors (R1, R2 and R3) were started up and operated at an organic loading rate of 1.40 kg COD m(-3) d(-1) with simple soluble feed substrate (i.e. jaggery) to obtain similar performance before application of shock. The shock was applied by suddenly changing feed substrate to reactors R2 (from jaggery to cerelac - a complex suspended type feed) and R3 (from jaggery to neutralized acetic acid) while keeping reactor R1 as the control. The immediate impact of sudden change in feed substrate was observed in reactor R3, which showed reduced gas production rate, while no visible impact on reactor R2 was observed for gas production rate. The performance of reactor R2 deteriorated marginally in respect of biogas production during the shock period. However, performance of reactor R3 deteriorated very much during the shock period: gas production ceased completely and indicated build-up of high levels of volatile fatty acid. Lastly, reactors R2 and R3 were restored to jaggery feed substrate. Reactor R3 responded by producing biogas along with the other two reactors. It was observed that all three reactors started giving steady performance within 2 weeks of operation, which was similar to pre-shock conditions. This study indicated that extreme caution is needed when there is a possibility of sudden stress/shock conditions due to complete change in feed substrate, even though all other operational and environmental parameters are planned to be maintained at pre-shock conditions. (c) 2006 Society of Chemical Industry.