Mechanochemical and mechanical alloying processes take place between colliding surfaces in the heavy container of a ball mill, where the in situ examination of the reaction mechanism is extremely challenging. As shown in this paper, useful indirect information can be obtained from detailed analysis of the reaction kinetics. A shaker mill with a single ball was used, so that time could be replaced with the number of collisions as the variable of kinetics. A simple stochastic model was developed that is capable of describing the kinetics of gradual mechanochemical reactions and the variation of physical properties such as grain size. The kinetic constant is directly related to the fraction of powder processed in a single collision, and its value indicates that only a few micrograms of powder are processed in a single collision. Measuring the kinetic constant as a function of impact energy revealed that a minimum impact energy, on the order of a few hundreds of a Joule, is needed to initiate chemical change. The model was also applied to the self-sustaining reaction between Ti and graphite. In that case, the critical number of collisions required for ignition characterizes the speed of mechanical activation.