A current knowledge gap in hydrogen safety research concerns the influence of mixture non-uniformity on explosion processes. Spacial mixture composition gradients are likely to form in real-world accident scenarios. In this work, the influence of such gradients on flame acceleration is experimentally investigated in an entirely closed channel at laboratory scale. Experiments show that concentration gradients can lead to significantly stronger flame acceleration compared to homogeneous mixtures. Optical measurements reveal that flame shapes differ between homogeneous and inhomogeneous mixtures in the unobstructed channel configuration. An enlargement of flame surface area in inhomogeneous mixtures leads to a higher integral reaction rate, which in turn supports flame acceleration. In obstructed channel configurations, however, mixture properties dominate flame propagation. An analytical model based on the integral balancing of mixture properties is proposed and validated by means of experimental data.