An investigation was made of the effect of environmental gases and pressures on the ignition of an ammonium perchlorate propellant. In addition, ignition studies were conducted on the individual propellants ingredients. The results indicate that the major exothermic reaction responsible for rapid convergence to steady state deflagration occurs in the gas phase immediately adjacent to the propellant surface. The shortest ignition times were obtained with oxygen as the environmental gas. The ignition times of both propellant and inert formulation were the same in this environment. Ignition times of the propellant in nitrogen were longer than in oxygen but approximately the same as the ignition time of pressed pellets in methane. The ignition times of the propellant in inert atmosphere was found to increase with decreasing molecular weight of the environmental gas. At the higher flux densities the ignition time was found to be dependent on the environmental pressure with ignition times increasing with decreasing pressure. These results are discussed in terms of a simple ignition model with (a) no chemical heating, (b) chemical heating in the condensed phase and (c) chemical heating in the gas phase.