A terrible mass explosion of about 400 tons of ammonium nitrate (AN) occurred in Toulouse AZF plant on 21 September 2001. According to the review of the accidental investigation, there may be resources of impurities to be introduced into the mass of ammonium nitrate. In this study, several chloride salts, including ammonium chloride, barium chloride, calcium chloride, sodium chloride and potassium chloride, were added into ammonium nitrate at concentrations of 0.05, 0.1, 0.5, 1 and 2%, respectively. A differential scanning calorimeter and a heat flux calorimeter C80 with higher sensitivity were used to measure the decomposition of ammonium nitrate and its mixtures with these impurities. All the results demonstrated that the addition of a small account of chloride gives rise to a significant decrease in the initial decomposition temperature of AN by nearly 100degreesC and the magnification of heat release from the reaction. The intensity of the decomposition of ammonium nitrate with and without impurities was also examined by a modified pressure vessel test (MCPVT), in which the rate of pressure rise of AN in the presence of chlorides in the MCPVT was multiple. It implied that, with the addition of impurities, the decomposition of AN is more intense and occurs at lower temperatures. However, in the BAM 50/60 steel tube test, no detonation was observed for either pure AN or contaminated AN under the test conditions. However a detonation is predicted under more severe conditions.