The changes in specific surface area and structure disorder of mechanically activated arsenopyrite were investigated. The rate of nonoxidative decomposition of mechanically activated arsenopyrite was increased almost 10-times when compared with nonoxidative decomposition of a non-activated sample. An empirical linear relationship was found (r=0.996) between the rate constant of decomposition and the ratio of specific surface to transmission of the absorption band of arsenopyrite at\(\bar \nu = 370 cm^{ - 1} \). This relationship enables us to arrange the reaction 4FeAsS→4FeS+As4 among structure-sensitive reactions.