Mesophilic and thermophilic digestion of primary and activated sludge was investigated in lab scale reactors. Mixed primary and activated sludge was digested in one experiment and activated sludge was digested alone in another. The same gas yield, COD and dry solids reduction was obtained with both mesophilic and thermophilic digestion. However, more dissolved COD was found after thermophilic digestion. A gas yield of 370 and 225 l/kg dry solids was found for mixed sludge and activated sludge respectively. The filterability of the sludge decreased after thermophilic digestion, but a pre-treatment with addition of ferric chloride or elutriation with water prior to addition of polymer provided an acceptable filterability. Thermal hydrolysis at 180ºC was tested as a pre-treatment prior to the digestion. This increased the gas yield by 22% for mixed sludge and 24% for activated sludge. Thermal hydrolysis further seemed to improve the dewaterability of the sludge after digestion.