In this work numerical simulations of hydrogen/air mixing are conducted which demon strate the generation of spatial pressure gradients arising from the diffusion of species (hydrogen versus air) of high molecular weight disparity Convective velocities arising as a result of the spatial gradients have been evaluated with respect to molecular weight ratio between stratified layers in coflow and stationary diffusion layers The implications of results for the spatially constant thermodynamic pressure assumption for low Mach number flows are discussed Additionally the impact of the choice of diffusion model on the results has been assessed (C) 2010 Professor T Nejat Veziroglu Published by Elsevier Ltd All rights reserved