Microbial activity in granular activated carbon (GAC) has the potential to extend the service life of GAC beds through in situ biological regeneration of sorption sites. Bioregeneration with phenol and paranitrophenol (PNP) was examined over the concentration range of 20–100μg/L and was measured using radiochemical analytical techniques. Bioregeneration ranged from 5 to 22 percent over a 10‐day period and typically showed a lag phase, followed by rapid bioregeneration, and finally a fairly constant, much lower rate. Differences in bioregeneration rate as a function of column position were slight with phenol as the substrate but substantial with PNP, for which bioregeneration was greatest at the influent end and smallest at the effluent end. The experimental results, in combination with mathematical modeling, suggest that bioregeneration can significantly affect the removal of low concentrations of synthetic organic chemicals.