The impacts of transient overloads on the performance of a laboratory-scale anaerobic ammonium oxidation (ANAMMOX) anaerobic baffled reactor was studied by increasing the substrate concentration or inflow rate to 1.5-3.0 times above normal values. These shocks, with the exception of the highest substrate shock, weakened the nitrogen removal efficiency (NRE) but improved the nitrogen removal rate by 0.01-0.18 g l(-1) h(-1). The communities and the location of the sludge may be altered by distinct types of shocks. The substrate vibration data showed that the reactor was unresponsive to hydraulic shocks but sensitive to substrate shocks and the former compartments were more susceptible to the shocks. In the inhibition period, the pH and NRE of the reactor were related to the residual ammonium and free ammonia (FA) and FA was a factor in the reactor fluctuations. The Gaussian model proposed to describe the shocks response fits the experimental data well. (C) 2012 Elsevier Ltd. All rights reserved.