Decolorization and mineralization of reactive dyes by intimately coupled TiO2-photocatalysis and biodegradation (ICPB) on a novel TiO2-coated biofilm carrier were investigated in a photocatalytic circulating-bed biofilm reactor (PCBBR). Two typical reactive dyes—Reactive Black 5 (RB5) and Reactive Yellow 86 (RY86)—showed similar first-order kinetics when being photocatalytically decolorized at low pH (∼4–5) in batch experiments. Photocatalytic decolorization was inhibited at neutral pH in the presence of phosphate or carbonate buffer, presumably due to electrostatic repulsion from negatively charged surface sites on TiO2, radical scavenging by phosphate or carbonate, or both. Therefore, continuous PCBBR experiments were carried out at a low pH (∼4.5) to maintain high photocatalytic efficiency. In the PCBBR, photocatalysis alone with TiO2-coated carriers could remove target compound RB5 and COD by 97% and 47%, respectively. Addition of biofilm inside macroporous carriers maintained a similar RB5 removal efficiency, but COD removal increased to 65%, which is evidence of ICPB despite the low pH. ICPB was further proven by finding microorganisms inside carriers at the end of the PCBBR experiments. A proposed ICPB pathway for RB5 suggests that a major intermediate, a naphthol derivative, was responsible for most of the residual COD, while most of the nitrogen in the azo-bonds (NN) was oxidized to N2. Biotechnol. Bioeng. 2012; 109:884–893. © 2011 Wiley Periodicals, Inc.