This paper deals with the application of a two-phase anaerobic digestion process where the first phase operates at extreme thermophilic conditions (70 degrees C). The first reactor was fed with waste activated sludge and operated continuously at a hydraulic retention time of 1, 2, 3, and 5 days. Pretreated sludge was characterized by high concentrations of soluble COD (30-40% of the influent particulate COD) and VFA contents. Acetate, propionate, and isovalerate were the main compounds detected. The kinetic constant for the hydrolysis process was determined in 0. 17 day(-1). Batch tests for the following anaerobic digestion and biogas production showed how the pretreated sludge determined better performances in terms of biogas production. The gas production showed increases in the range 30-50% for pretreatments of 2-3 days compared to the mesophilic and thermophilic single-stage tests. A calculation for a 100 000 people equivalent wastewater treatment plant showed that the increased biogas production allowed maintenance of the thermophilic conditions in both the first and second stages of a two-phase process and recovery of the investment costs in some 3-4 years.