1. Ignition of solid propel Iants of the ammonium perchlorate type has been accomplished in a shock tube in times of the order of a millisecond, despite the relatively modest temperature, less than 200°C, developed at the surface of the propel Iant. The explanation of this performance rests on a gas phase theory of ignition, entirely different from the solid phase theory that had been developed by Altman in 1950 and Hicks in 1954 and which has been accepted by others since that date. The evidence in favor of the new theory lies mainly in the observed strong effect of oxygen in the ignition gas on the ignition delay. A further indication of its validity is that it predicts the correct order of ignition delay (about I millisecond) in these experiments, with no adjustable parameters or "fudge" factors and no assumed "global" activation energy. The ignition process at high surface temperature levels can also be explained by the gas phase ignition theory, as illustrated by the correct prediction of the previously inexplicable data of others.