A protocol proposed is described here to characterize for the conversion from mesophilic to thermophilic conditions in a pilot-scale anaerobic digester that operates with municipal mixed sludge. Furthermore, the performance of thermophilic operation relative to the previous mesophilic operating status was evaluated. The performance was evaluated in terms of a number of parameters that included organic removal rate (ORR) (kgVS/m(3)center dot d and kgCOD/m(3)center dot d), biogas and volumetric methane production rate (m(3)/m(3)center dot d), pH, total acidity (mg acetic acid/L) and acidity/alkalinity relationship. The digester was initially operated with an organic loading rate (OLR) of 1.26 kgVS/m(3)center dot d and a solids retention time (SRT) of 27 days under mesophilic conditions (35 degrees C). The solids destruction efficiency was found to be 54.3%, while the volumetric biogas production in the digester reached 0.36 in m(3)/m(3)center dot d. The strategy selected for the conversion from mesophilic to thermophilic digestion involved slowly increasing the temperature of the digester (0.38 degrees C/d) until it reached 43 degrees C In this way, the temperature of the digester was raised from 43 to 45 degrees C and then operated at a constant 45 degrees C. The performance parameters at this temperature indicated that the digester was unstable. For this reason the OLR was decreased until feeding was suppressed. The reactor operated at 45 degrees C for 32 days, and the temperature of the digester was then raised from 45 to 50 degrees C (without feeding). The temperature was subsequently raised to 50-52 degrees C with the system operating at variable SRT (65-52 days), and finally, the temperature was increased at a rate of 0.13 degrees C/d until it reached 55 degrees C At thermophilic conditions (55 degrees C), the OLR studied was 1.48 kgVS/m(3)center dot d (SRT: 27 days), and under these conditions the solids destruction efficiency was 53.3% VS, and the biogas produced in the digester reached 0.32 m(3)/m(3)center dot d. (c) 2005 American Institute of Chemical Engineers.