AbstractEquations for solid‐state decompositions which are controlled by the phase‐boundary movement and nucleation have been examined using ammonium perchlorate/polystyrene propellant decomposition at 503 K and 533 K. It was found that 3 different equations governed by the nucleation process show a good fit of data at these temperatures. However, the best fit was obtained for the following Avrami‐Erofeev equation, [‐In (1 ‐ α]1/4=kt.