The detonation failure process, specifically where a transient reaction occurs in an overdriven explosive due to a sub-critical charge diameter, has remained relatively unexplored, particularly regarding its relationship to shock initiation mechanisms. Detonation failure, here defined as a weakly supported shock wave traveling with a decaying velocity, of aluminized ammonium nitrate (AN/Al) was considered to investigate the relationship between the detonation failure process and shock sensitivity. Previous work has shown that the Al additive particle size alters the failure diameter, a measure of shock sensitivity, of AN/Al. Microwave interferometry was used to measure the transient shock wave velocity in the explosive system, which was lightly confined at a diameter below its critical diameter, in response to an overdriven shock insult in a small-scale experiment. An array of mixture ratios and particle size distributions of the explosive system was used to vary the sensitivity of the explosive and to facilitate exploration of the relationship between the shock velocity decay rate, initiation mechanisms, and large-scale shock sensitivity. Resulting shock velocity profiles in AN/Al indicate that micron-sized and larger Al particles can contribute to the detonation process. It is concluded that the rate at which the transient shock velocity approaches that of a compressive wave with no supporting reaction corresponds to the relative shock sensitivity of the system; it is therefore proposed that the measured shock velocity decay rate of an overdriven subcritical diameter charge of AN/Al reveals quantifiable information about the relative shock sensitivity of a large-scale charge. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.