This study investigated the photocatalytic degradation of acetaminophen (APAP) in TiO2 suspended solution under a 250W metal halide lamp. The influence of some parameters on the degradation of acetaminophen was studied and described in details, such as initial APAP concentration, initial pH value and TiO2 dosage. After 100 min irradiation, about 95% of APAP is decomposed in the 1.0 g L-1 TiO2 aqueous solution with an initial concentration of 100 mu mol L-1. The effect of adsorption at three different pH values has also been analyzed and it has been conducted that pH 3.5, at which APAP was readily adsorbed also degraded at a faster rate. Reaction rate at pH 6.9 and pH 9.5 was 2.84 and 2.96 mu M min(-1), respectively. Direct hole (h(+)) oxidation and ipso-substitution was found to be the main initial step for APAP degradation. Main reaction intermediates and products were identified by GC/MS analysis. The mechanism of acetaminophen photocatalytic degradation in TiO2 suspended solution was studied not only experimentally but also theoretically by calculating the frontier electron density of APAP. The results obtained indicated that TiO2 photocatalytic degradation is a highly effective way to remove APAP from wastewater and drinking water without any generation of more toxic products. (c) 2008 Elsevier B.V. All rights reserved.