A universal empirical relation is derived to predict the reaction delay times of aluminum particles as a function of particle diameter and gas temperature in explosives. The predicted delay times are shown to be in reasonable agreement with experimental data obtained in both low temperature and high temperature explosives. A convective heat transfer model is used to estimate the surface temperature of the solid aluminum particles. The reaction delay time of aluminum is shown to be close to the time when the particle surface reaches the pressure dependent melting point of aluminum.