In this study, photocatalytic degradation of 2,4,6-trimethylphenol (TMP), 2,4,6-trichlorophenol (TCP), 2,4,6-tribromophenol (TBP), 2,4-dimethylphenol (DMP), 2,4-dichlorophenol (DCP) and 2,4-dibromophenol (DBP) has been studied by TiO2/UV. Although degraded phenolic compound concentration increased by increasing initial concentration photocatalytic decomposition rates of di- and tri-substituted phenols at 0.1-0.5 mM initial concentrations decreased when the initial concentration increased. The fastest degradation observed for TCP and the slowest for TMP. Photodegradation kinetics of the compounds has been explained in terms of Langmuir-Hinshelwood kinetics model. Degradation rate constants have been observed to be extremely depended on electronegativity of the substituents on phenolic ring. Degradation rate constant and adsorption equilibrium constant of TCP were calculated as k0.0083 mM min(-1) and K9.03 mM(-1). For TBP and TMP the values of k and K were obtained as 0.0040 mM min(-1) 19.20mM(-1), and 0.0017 mM min(-1), 51.68 mM(-1) respectively. Degradation rate constant of DBP was similar as DCP (0.0029 mM min(-1) for DBP and 0.0031 mM min(-1) for DCP) whereas adsorption equilibrium constants differed (48.40 mM(-1) for DBP and 30.52 mm(-1) for DCP). K and k of DMP found as 83.68 mM(-1) and 0.0019 mM min(-1), respectively. The adsorption equilibrium constants in the dark were ranged between 1.11 and 3.28 mM(-1) which are lower than those obtained in kinetics. Adsorption constants have inversely proportion with degradation rate constants for all phenolic compounds studied. (c) 2004 Elsevier B.V. All rights reserved.