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Abstract: Polysulfone (PS) hollow fiber membranes impregnated with granular activated carbon (GAC) were fabricated, characterized, and tested for the cometabolic biotransformation of 4-chlorophenol (4-cp) in the presence of phenol. The spinning solutions contained PS, N-methyl-2-pyrrolidone (NMP) and various weight ratios of GAC (GAC:PS of 0, 1:4, and 1:2). The internal coagulants investigated were water or NMP individually, as well as a 50/50 mixture of water and NMP. SEM observations show that the morphology of the hollow fiber was affected by the choice of the internal coagulant and the amount of GAC incorporated. A five-layer structure resulted when the internal coagulant was water or the 50/50 water/NMP mixture while a more open, porous three-layer structure resulted from using NMP as internal coagulant. The fiber surfaces were also rougher and jagged with higher GAC content. Adsorption/desorption studies were performed and the data corroborated the Freundlich isotherm. The membranes were tested for biodegradation of 600mg/L phenol and 120mg/L 4-cp for four times consecutively to demonstrate reusability and a fifth at higher concentrations of 1000mg/L phenol and 200mg/L 4-cp. Complete biotransformation of the substrates was accomplished in all the five runs in all the nine membrane types fabricated. SEM visualizations at the end showed that the immobilized cells grew in large cell floes preferentially on the external surfaces of the membranes. The removal rate of the immobilized cells systems with 1:2 GAC using NMP as internal coagulant was the highest of the systems because of the more open membrane structure and higher GAC content. [Copyright &y& Elsevier]