This paper examines the effect of gradients in reactivity on the propagation of detonations in a relatively large (similar to 1 m high) channel by studying detonation transmission from stoichiometric to fuel-lean mixtures of methane and air. The numerical model solves the fully compressible, reactive Navier-Stokes equations. The chemical heat release, mass diffusion and species-production rates are modeled by a new version of the chemical-diffusive model (CDM), calibrated to reproduce flame and detonation properties over a range of equivalence ratios. One and two-dimensional simulations are performed for three different reactivity gradients, corresponding to a steep, intermediate, and shallow gradient in reactivity. In one dimension, detonation transmission across all reactivity gradients results in detonation failure, as the detonation wavefront decouples into a shock and a flame. This is not the case in two dimensions, where the dynamics of detonation transmission and failure are far more complex due to the presence of transverse waves and the shock collisions associated with them. Successful transmission of the detonation in two dimensions depends on the steepness of the reactivity gradient, the local strength of the detonation, and the interaction of the detonation with the Taylor expansion fan. In multidimensions, weaker parts of the detonation decouple into separated shocks and flames, but the overdriven parts of the wavefronts survive. The result is partial failure at the front. Such partially failed detonations, however, can develop extremely high pressure overdriven transverse detonations in their reaction zones. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.