The objectives of this research included evaluation and comparison of the occurrence of the biological removal of phenol adsorbed onto granular activated carbon (GAC), under anaerobic methanogenic conditions, and at different phenol gradients between the GAC and the bulk liquid. The scope included monitoring the degradation of radiolabeled phenol and production of radiolabeled methane in three column reactors operated in parallel under different gradients of phenol. Three columns were operated in semi-continuous mode at 35C. GAC was pre-loaded with radiolabeled phenol at an equilibrium concentration of approximately 100 mg/L. During Phase 1, phenol was fed daily at dosages that (a) increased the phenol concentration, (b) maintained approximately constant, and (c) decreased the phenol concentration, respectively in each of the three columns to establish different gradients between GAC and bulk liquid. The feeding scheme was reversed for Phase 2 in each of the columns. Conventional bottle-point procedures and desorption procedures were used to develop isotherms under anaerobic conditions.