Biochemical methane potential assays were performed to assess the influence of ultrasonic pretreatment on the temperature-phased anaerobic digestion (TPAD) of waste-activated sludge (WAS). Ultrasound (specific energy = 3380 kJ kg(-1) TS) was applied to the WAS before the thermophilic stage or to the effluent after the thermophilic stage. In addition, a control system without pretreatment was also carried out. No significant differences were found in the overall performance of the TPAD process, but different behaviours were observed between the thermophilic and mesophilic stages. Total methane production was enhanced by more than 50% and the volatile solid removal increased by 13% in comparison to the TPAD control process. Finally, a previously defined kinetic model was applied successfully to the experimental data and showed an excellent fit. (C) 2014 Elsevier B.V. All rights reserved.