The effects of various anions, Cl-, ClO4-, SO42-, NO3-, HCO3-, H2PO4-and C2O42-, on the photocatalytic and photoelectrocatalytic degradation of reactive Brilliant Orange K-R have been investigated in a packed-bed photoelectrocatalytic reactor. It was found that the nature and concentrations of these inorganic anions significantly affected the photocatalytic and photoelectrocatalytic degradation performance of the reactive dye. The results indicated that the external electric field was successfully applied to improve the photocatalytic efficiency of reactive Brilliant Orange K-R in the presence of Cl-, especially at higher concentrations, while other inorganic anions displayed more or less negative effects on the degradation of the dye. The strongest inhibition effect on photocatalytic and photoelectrocatalytic degradation of the dye was observed in the presence of HCO3- ions.(C) 2004 Society of Chemical Industry.