Phenol is a common wastewater contaminant from various industrial processes, including petrochemical refineries and chemical compounds production. Due to its toxicity to microbial activity, it can affect the efficiency of biological wastewater treatment processes. In this study, the efficiency of an Anaerobic Sequencing Batch Reactor (ASBR) fed with increasing phenol concentrations (from 120 to 1200mgL-1) was assessed and the relationship between phenol degradation capacity and the microbial community structure was evaluated. Up to a feeding concentration of 800mgL-1, the initial degradation rate steadily increased with phenol concentration (up to 180mgL-1d-1) and the elimination capacity remained relatively constant around 27mg phenol removedgVSS-1d-1. Operation at higher concentrations (1200mgL-1) resulted in a still efficient but slower process: the elimination capacity and the initial degradation rate decreased to, respectively, 11mg phenol removedgVSS-1d-1 and 154mgL-1d-1. As revealed by Denaturing Gradient Gel Electrophoresis (DGGE) analysis, the increase of phenol concentration induced level-dependent structural modifications of the community composition which suggest an adaptation process. The increase of phenol concentration from 120 to 800mgL-1 had little effect on the community structure, while it involved drastic structural changes when increasing from 800 to 1200mgL-1, including a strong community structure shift, suggesting the specialization of the community through the emergence and selection of most adapted phylotypes. The thresholds of structural and functional disturbances were similar, suggesting the correlation of degradation performance and community structure. The Canonical Correspondence Analysis (CCA) confirmed that the ASBR functional performance was essentially driven by specific community traits. Under the highest feeding concentration, the most abundant ribotype probably involved in successful phenol degradation at 1200mgL-1 was affiliated to the Anaerolineaceae family.