The kinetics of decomposition of polycrystalline and single crystal TIN3 were studied using the pressure-time method. Several crystal sizes were employed and the extent of an initial decomposition reaction was roughly proportional to their specific surface areas. An induction period followed the initial reaction and its duration increased with increase in crystal size. Finally, a maximum, linear rate of decomposition developed which was proportional to specific surface area. Microscopy revealed that decomposition began at preferred faces and proceeded in preferred crystal directios. A tendency to bore holes of uniform depth and constant cross-sectional area accounted for the linear kinetics.