The method of volume averaging is used to derive the diffusive mass transfer boundary conditions for transport between the micro-pores (co-region) and the fluid in the macro-pores (q-region) in a catalyst pellet. In this configuration, the mass jump boundary condition between the homogeneous regions takes the form -n-n(eta omega) center dot (D(gamma)del < c(A gamma)>(gamma)(eta)) + n(eta omega) center dot (epsilon(gamma)D(omega) center dot del < c(A gamma)>(gamma)(omega)) = K-eff < c(A gamma)>(gamma)(omega), where K-eff is the effective reaction rate coefficient at the inter-region. In this study, a closure is derived in order to predict this average jump coefficient as a function of the microstructure of the porous layer and the Thiele modulus. The jump coefficient predicted for three inter-region structures is presented. (c) 2005 Published by Elsevier Ltd.