In this paper we apply novel experimental and computational methods that investigate the contribution of cracks and cavities to diffusion in catalytic coating (washcoat). A sample of commercial catalytic monolith for automotive exhaust gas after treatment is scanned by X-ray micro tomography (mu CT) to obtain realistic 3D images of the coated layer. In the tested sample the cracks and cavities represent about 20% of the coated layer volume. A 3D diffusion is then simulated within sections of the virtually reconstructed porous structure. Benchmark simulations performed for the coating without any cracks or cavities (i.e. containing only standard meso-and macro-pores) indicate that the overall effective diffusivity is approx. 1.7 times lower in absence of these structural defects and the difference increases with temperature. The effect of cracks and cavities is further tested in combined reaction and transport simulation and corresponding effectiveness factors are evaluated. Finally, spatially averaged reaction rates are employed in multi-scale simulations of light-off curves that reveal the impact of the structural defects in the catalytic coating on overall performance of the monolith reactor. (C) 2013 Elsevier B.V. All rights reserved.