The adsorption and photocatalytic degradation abilities of bismuth catalyst (BiOCl0.75I0.25) in water are tested under simulated solar light by using p-hydroxyphenylacetic acid (p-HPA) as chemical pollutant. On the basis of the strong adsorption of p-HPA on the catalyst surface, this work aims to investigate the interfacial mechanism in the heterogeneous system through adsorption/desorption experiments by using competitive-anion desorption with phosphate anions. The key factors affecting the degradation kinetics of p-HPA such as pH and dissolved oxygen concentration are investigated. We find that solution pH significantly affects the adsorption and degradation rates depending on the zeta potential of catalyst and molecular form of p-HPA. p-HPA shows maximum adsorption at pH 4.5 and faster degradation rate at pH 3.0. Dissolved oxygen concentration is a key factor affecting pollutant removal because of the formation of hydroperoxide/superoxide radical anion couple (HO2/O2−). These radicals, which are the main reactive species involved in the reaction after irradiation of catalyst, are detected using competition-kinetics approach and selective radical probes.