Biodegradation of resorcinol, catechol and hydroquinone under anaerobic conditions was studied in identical upflow fixed film-fixed bed reactors. Kinetic constants; V-max (maximum substrate utilization rate) and K-s (Monod's half saturation constant) were determined for the three compounds using Lineweaver-Burk plot. V-max for resorcinol was highest, followed by catechol and then by hydroquinone. When both resorcinol and catechol were fed to the resorcinol-acclimated reactor, resorcinol degradation was inhibited by catechol. The inhibition was of the uncompetitive type and V-max for resorcinol was reduced by catechol.