Siliceous zeolites are thermally and chemically stable microporous materials that are promising in membrane form for such high temperature (>773 K) processes as hydrogen separation after coal gasification, as well as other separation processes including air separation for oxy combustion, post-combustion carbon dioxide separation and natural gas purification from carbon dioxide. The current study employs molecular dynamics and grand canonical Monte Carlo approaches to predict single-component gas diffusivities and adsorption isotherms of H-2, CO2, CH4, O-2 and N-2 in decadodecasil 3R (DD3R) zeolite at 273-1073 K. The gas diffusivities and adsorption loadings determined in this study enable prediction of separation characteristics of DD3R employed as a membrane material at relevant process conditions. These separation characteristics include gas fluxes, permeances and selectivities to qualify the material as a membrane for the specified processes. It is observed that adsorption of H-2, CO2, CH4, O-2 and N-2 in DD3R plays a more significant role in overall mass transport and separation selectivities at lower temperatures as compared to their diffusivities. While hydrogen adsorption is relatively low at all temperatures, adsorption of all gases in DD3R is insignificant at elevated temperatures (>473 K), resulting in a dominant contribution of molecular diffusivity to mass transport and gas separation selectivity. (c) 2012 Elsevier B.V. All rights reserved.