Measuring Effective Diffusivity in the pores of the catalytic washcoat of a monolith reactor is a challenging task. In this work, Spaci-MS technique has been used to measure effective diffusivity in the porous washcoat, and through the porous walls of the cordierite monolith substrate. A mixture of a tracer gas (in this case O-2) was introduced in a center (feed) channel along with a carrier gas, and the axial profile of tracer gas was measured in the neighboring channels. The axial profile of the tracer gas in the neighboring channels depends on the diffusivities in the gas phase, in the washcoat layer and through the substrate wall. A mass balance was made around the 1-adjacent channel to compare the calculated tracer profile with the measured profile, and used to get the value of effective diffusivity. Experiments were done on an uncoated (blank, substrate only) monolith to measure the effective diffusivity in the substrate, and then experiments were repeated for a washcoated monolith to measure the effective diffusivity in the catalytic washcoat layer. The experiments were also repeated at different temperatures to get the temperature dependence of the observed effective diffusivity. The results showed that the monolith substrate was in the bulk diffusion region and the washcoat exhibited Knudsen diffusion. Effective diffusivity of O-2 in substrate was calculated to be 9.28 x 10(-6) m(2)/s and in washcoat to be 2.2 x 10(-6) m(2)/s at 100 degrees C. (C) 2016 Elsevier B.V. All rights reserved.