The solid-state phase transitions of the KNO3–NH4NO3 solid solutions have been determined by high temperature X-ray diffractometry, and lattice parameter calculation has also been performed. Ammonium nitrate (AN) is of great use for gas generators of automobile air bag systems. The X-ray diffraction results showed the single (AN) phase III from 5% to 20% KNO3 in NH4NO3 and up to 373 K, which is the important temperature range for the air bag gas generator applications. The X-ray diffraction patterns of the low temperature KNO3 phase (KN II) are from 92% to 100% KNO3 composition range and up to 393 K temperature. The high temperature KNO3 phase (KN I) showed very broad composition range from 20% up to 100% KNO3 at various temperature ranges. The lattice parameters of the NH4NO3-rich (AN III) and KNO3-rich (KN II and KN I) solid solutions have been calculated at different temperature range. The volumes of AN III phase decrease from 0.3201(4) to 0.3166(1) nm3 at room temperature and from 0.3250(6) to 0.3215(3) nm3 at 373 K as the compositions increase from 5% to 20% KNO3. The lattice constants of the hexagonal KN I phase show that there is no significant change in a direction when the temperature increases. Details of X-ray results, lattice expansions, and equations during heating are presented.