Gaseous detonations propagating through a layer of reactants that is bounded by an inert gas were simulated by solving the two-dimensional reactive Euler equations. A single-step chemical reaction model was used with thermochemical properties that are representative of a highly reactive fuel-oxidizer mixture, such as stoichiometric ethylene and oxygen. A series of cases with varying acoustic impedance ratios between the inert and reactant gases, Z, were studied to explore the influence of acoustic impedance mismatch on the propagation of a detonation through the reactant layer. The detonation failed when Z similar to 1 due to a loss of triple points at the interface. The detonation propagated stably when Z is high and the impedance of the inert gas is much higher than the reactants. Reflected shocks are produced from the interaction of the Mach stem of a detonation cell with the interface between the reactant and inert gases. These reflected shocks, in turn, detach and generate new triple points that are necessary to propagate the detonation. The detonation was also stable when the acoustic impedance of the inert gas is much lower than the reactants. A gas dynamic structure forms that involves a detached shock in the inert gas and a series of oblique shocks and slip lines in the reactants. A small local explosion is triggered when the Mach stem of a detonation cell interacts with the compressed reactants behind one of these oblique shocks. The resulting retonation wave produces a new Mach stem and a new triple point that leads to a stable detonation. These results suggest that the acoustic impedance and shock structure in the inert gas can have a significant influence on the stable propagation of a layered gaseous detonation. (C) 2017 The Combustion Institute. Published by Elsevier Inc. All rights reserved.