Cellulose is a key component of the biosphere and also the most important constituant of waste produced by our society. Understanding and optimizing its methanization is thus of considerable interest in the framework of green energy production policy. In order to better characterize functional communities of microbes associated to its degradation, a series of samples from MSW digesters were incubated with 13C-cellulose, 13C-glucose, 13C-acetate under mesophilic and thermophilic conditions. Physico-chemical parameters were analyzed and associated functional microbial diversities were characterized by rDNA-SIP. Totally, 1251 bacterial and 1312 archaeal 16S rDNA sequences were analyzed together with corresponding SSCP profiles. Then, specific FISH probes were designed and hybridized. Microorganisms affiliated with genus Acetivibrio and order Halanaerobiales were respectively the principal cellulose hydrolyzers under mesophilic and thermophilic conditions, whereas microorganisms affiliated with the family Porphyromonadaceae and the genus Clostridium were the main glucose fermenters. Large amount of microorganisms affiliated with class Clostridia, but distantly related to any hitherto cultured strain, were identified as functional groups in all degradation experiments. Syntrophic oxidation of acetate took place under both temperature conditions and might be catalyzed by Pseudomonas-related microorganisms under mesophilic conditions. Incubation temperature shaped the general microbial diversity while the added substrates played an important role on the functional diversity. A high functional versatility was evidenced under thermophilic conditions which could contribute to a more rapid and efficient cellulose degradation. This study sheds light on networks of uncultured microbes catalyzing the methanization of the most abundant chemical renewable energy source on earth.