The present study reports a novel treatment concept combining the redox-mediating capacity of immobilized humic substances with the biodegrading activity of anaerobic sludge for the simultaneous removal of two representative pollutants of textile wastewaters (e.g., phenol and Reactive Red 2 (RR2)) in a high-rate anaerobic reactor. The use of immobilized humic substances (1 g total organic carbon (TOC) L-1, supported on an anion exchange resin) in an upflow anaerobic sludge blanket (UASB) reactor increased the decolorization efficiency of RR2 (similar to 90 %), extent of phenol oxidation (similar to 75 %), and stability as compared to a control UASB reactor operated without immobilized humic substances, which collapsed after 120 days of dye introduction (50-100 mg L-1). Increase in the concentration of immobilized humic substances (2 g TOC L-1) further enhanced the stability and efficiency of the UASB reactor. Detection of aniline in the effluent as RR2 reduction product confirmed that reduction of RR2 was the major mechanism of dye removal. This is the first demonstration of immobilized humic substances serving as effective redox mediators for the removal of recalcitrant pollutants from wastewater in a high-rate anaerobic bioreactor. The novel treatment concept could also be applicable to remove a wide variety of contaminants susceptible to redox conversion, which are commonly found in different industrial sectors.