Ammonium nitrate-fuel oil (anfo) blasting agents may undergo an exothermic reaction when they contact warm ore bodies that contain pyrite and its weathering products. The reaction is very fast at temperatures above 80 deg c. Urea, when present in sufficient amounts, inhibits this reaction at temperatures below 180 deg C but urea-ammonium nitrate mixtures are very hygroscopic and, in certain climates, may be difficult to handle in the field. Recently, the Bureau investigated the reaction-inhibiting qualities of five compounds. Mixtures of anfo, pyrite, weathering product, and prospective inhibitors were slowly heated, under conditions simulating a hot borehole, to determine prospective inhibitor capability at tempteratures up to approximately 175 deg c. Two organic compounds, phthalimide and xanthine, similar in structure to urea, failed to inhibit the reaction. Zinc oxide and calcium carbonate were found inferior to urea, through zinc oxide raised the onset of reaction from 80 deg c to about 140 deg c. Only magnesium oxide was found to be similar to urea in its inhibitory ability at temperatures below 175 deg c.