The degradation of the organophosphorus pesticide diazinon by means of sono-Fenton and sono-Fenton-like systems was investigated in this study. The effects of a combination of transition metals, the concentration of Fenton's reagent and temperature on diazinon degradation were examined. An initial concentration of 50 mg L-1 diazinon was used in this study as a model solution. For the sono-Fenton and sono-Fenton-like experiments, an ultrasonic probe at a frequency of 20 kHz was employed. The optimal experimental conditions were determined to be as follows: 20 mg L-1 Fe2(+), 150 mg L-1 H2O2, 25 degrees C and pH 3. According to the results, the diazinon removal efficiency after 60 min was 98.3%, and the mineralization efficiency was 29.9%. It was also found that the toxicity of the diazinon solution was efficiently reduced by a sono-Fenton process. The most important mechanism of degradation was the substitution of sulfur by oxygen on the P=S bond in diazinon through oxidation. (C) 2014 Elsevier B.V. All rights reserved.