This CONACYT sponsored research focused on studying the anaerobic biochemistry of complex microbial communities living in anaerobic digesting sludge and feeding on phenolic compounds. Phenols in the environment are sometimes unwanted due to their potential toxic and inhibitory effects. For instance, when released into wastewater, phenols could cause a reduction on biogas production from anaerobic digesters. In this respect, the anaerobic fermentation of aromatic compounds is challenging due to thermodynamic limitations, toxic effects and the synergistic and syntrophic associations established between microorganisms and their environment. My research aims were to develop a fermentative-methanogenic consortia degrading phenol; to study the anaerobic treatment of phenol in a two stage anaerobic digester (bioreactors); to study the toxicity and biodegradability of seven aromatic compounds and to correlate biomass with chemical composition. (to be continued).