The present paper addresses the ignition problem of a one-dimensional unsteady diffusion layer of fuel and oxidizer, undergoing volumetric expansion. The problem is applied to shock induced diffusion-ignition of pressurized fuel jets that are released into an oxidizing atmosphere. Upon the sudden release of a pressurized gaseous fuel into the ambient atmosphere through a hole, a strong shock wave forms, driven by rapid expansion of the forming jet. The model follows the thin diffusion layer at the head of the jet in Lagrangian coordinates, with its rate of expansion dictated by the local pressure evolution of the surrounding gasdynamic flow. Following the analysis of Radulescu and Law, the latter can be calculated a priori before the ignition event. Hence, the expansion rate is prescribed as a source term in our calculations of the diffusion layer. The calculations, which are performed for hydrogen and air with realistic thermo-chemical data and transport properties of the chemical species, revealed the transient events leading to ignition in this unsteady diffusion layer. Furthermore, the calculations showed that when the rate of expansion was sufficiently strong, which may occur for releases through sufficiently small holes, ignition can be prevented. This illustrates the important role that gasdynamic expansion plays on ignition phenomena. The results of the present model are found to be in very good agreement with previous numerical and experimental results of transient jet release ignition. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.