The kinetics and mechanism of the thermal decomposition of ammonium iodate have been studied by measuring the loss in weight and the pressures of the gases evolved. Ammonium iodate decomposes at a measurable rate between 145-165° and explodes above 180°. The presence of ammonia, however, greatly increases its stability. The solid residue from the decomposition consists of a mixture of HI3O8 and I2O5. The reaction is deceleratory in character and is described by the contracting cube formula and a monomolecular decay equation. The value of the activation energy is 66 kcal/mole. In addition some important physical properties relating to the bonding and electronic structure of ammonium iodate are determined. The possible mechanisms of the decomposition are discussed and it is assumed that the decomposition proceeds by a proton transfer process.