Chlorine is widely used in drinking water treatment plants as a common disinfectant, and it also can effectively remove many pharmaceuticals and personal care products in water and wastewater. Two pyrazolone compounds (propyphenazone and aminopyrine) that have been frequently detected in aquatic environments were selected for investigation into their removal during free chlorine disinfection. The influencing factors on the removal of the pharmaceuticals were explored at initial pharmaceutical concentrations ranging from 0.1 to 1.25 mu M, chlorine dosages ranging from 14.08 to 28.17 mu M and pH values ranging from 3.0 to 9.0. The results showed high removal efficiencies of the two pharmaceuticals by free chlorine in the order of propyphenazone>aminopyrine. Their removal and reaction rate constants (k(obs)) were clearly dependent on the chlorine dosage and pH. High chlorine dosages accelerated the removal rates of both compounds. The highest removal efficiencies and the highest k(obs) were observed at neutral pH for propyphenazone (>90% for 24 s reaction) and slightly acidic pH for aminopyrine (>85% for 110 s reaction). In addition, the degradation of the two compounds by free chlorine and the structure characteristics of the oxidation byproducts were estimated by non-purgeable organic carbon (NPOC), ultraviolet (UV-Vis) spectroscopy and Fourier transform infrared spectroscopy (FT-IR) analysis. The results of the NPOC analysis indicated that chlorine cannot completely mineralize propyphenazone or aminopyrine. Chlorine atom substitution, dealkylations and ring-opening reactions may occur during propyphenazone or aminopyrine chlorination according to the UV-Vis and FT-IR analysis. (C) 2014 Elsevier B.V. All rights reserved.