Effects of mass transfer on photocatalytic activity were investigated using a recirculated feed solution (20 ~ 500 mL/min) in a UV/TiO₂ packed-bed photoreactor. The rate of phenol disappearance was significantly increased as the recirculation rate was raised up to 300 mL/min and reached a plateau beyond 400 mL/min. We confirmed that the degradation rate was influenced by the convective mass transfer of phenol from the bulk solution to the external surface of the catalysis particles. Other factors, such as solution pH and initial concentration, exhibited clear influences on the photocatalytic activity of the UV/TiO₂ packed-bed system. Assuming a constant intrinsic kinetic coefficient, the ratio of effective catalyst surface area to reactor volume, a_exp (cm²/cm³), was estimated to be on the order of 10^-3 by the re si stance-in-series model of rate constants. The obtained low a values indicate that only a small fraction of the surface area was effectively illuminated by the radiant source in the present reactor configuration and that improving the a value should be considered as one of the parameters for optimizing photoreactor design.