A comparison of two sets of mesophilic and thermophilic anaerobic sludges (one set from the treatment of palm oil mill effluent, the other produced during the treatment of coffee wastes) showed that the thermophilic sludges were more susceptible to sonication than their mesophilic counterparts. Extracellular polymers were extracted from each of the four samples. A comparison of the analytical data for the polymers with the sonication data suggested that the strength of the samples, as determined by sonication, was related to the amount of polymer and its composition. When sections of the samples were examined microscopically and the results compared with the sonication data, it appeared that the presence of filamentous methanogen, Methanothrix, gave a stronger granule than when Methanosarcina was dominant.