Display omitted •Ammonium nitrate and pyrite react to produce nitric oxide and nitrous acid.•Thermal decomposition is accelerated by NOx and nitrous acid catalysts.•Adsorption of NOx gas slows the decomposition of ammonium nitrate with pyrite.•Hydrotalcite adsorbs NOx gases to inhibit the thermal decomposition. The mechanism of the decomposition of ammonium nitrate fuel oil (ANFO) and of ammonium nitrate (AN) emulsions has been investigated. The formation of NO gas bubbles at the end of an induction period was observed, which led to the evolution of brown NO2 gas on contact with air. Inhibition of the decomposition reaction could be achieved by reducing the NO concentration in the gas phase, with inhibitors not in physical contact with the reaction mixture. NO adsorbing clay minerals (hydrotalcite) were effective inhibitors, indicating multiple mechanisms of inhibition. A central role is assigned to nitrous acid (HNO2): sequestering NO prevents its formation, base inhibitors keep it in the non-reactive form of the nitrite ion (pKa of HNO2∼3), and urea in acidic conditions decomposes it to harmless N2. Our results have implications in industries where NOx emissions can cause significant environmental problems for both aquatic and marine life as well as for human health.