Strategies for the transition of municipal sludge anaerobic digestion from mesophilic to thermophilic were assessed and the long-term stability and performance of thermophilic digesters operated at a solids retention time of 30 days were evaluated. Transition from 36 degrees C to 53.3 degrees C at a rate of 3 degrees C/day resulted in fluctuation of the daily gas and volatile fatty acids (VFAs) production. Steady-state was reached within 35 days from the onset of temperature increase. Transitions from either 36 or 53.3 degrees C to 60 degrees C resulted in relatively stable daily gas production, but VFAs remained at very high levels (in excess of 5000 mg COD/L) and methane production was lower than that of the mesophilic reactor. It was concluded that in order to achieve high VS and COD destruction and methane production, the temperature of continuous-flow, suspended growth digesters fed with mixed municipal sludge should be kept below 60 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.