In this study, changes in selective adsorption behavior and separation properties due to nanocasting process have been investigated, and for this purpose the nanocasting of ordered mesoporous silica (SBA-15) to its carbon replica (CMK-3) has been considered for the first time. The SBA-15 and CMK-3 possesses different sets of unique porous structure and morphologies, which enables them to demonstrate differences in adsorption preferences for VOC and gas molecules. Both materials were subjected to various characterization techniques, involving comparative morphological study with Transmission Electron Microscopy images, low temperature nitrogen adsorption, and XRD. To ascertain their adsorption properties, studies of vapor phase adsorption with three VOCs viz, ethanol, n-hexane, and toluene and studies of high pressure gas phase adsorption with four gases viz. CO2, C2H6, C2H4, and CH4 have been conducted. The isotherm data, obtained from these studies, was applied to different models of binary gas separations, to compare (i) the variation in selectivity with total applied pressure, at fixed composition and (ii) the variation in extent of separation with composition of binary mixture, at fixed pressure, i.e. 100 kPa. Results confirm that nanocasting of SBA-15 to CMK-3 involves changes in chemical as well as structural properties, which ultimately causes changes in adsorption preference, for example SBA-15 and CMK-3 shows quite opposite selectivity to a binary gas mixture of C2H6/C2H4. It was noticed that siloxane groups (-Si-OH) in SBA-15 play a key role in configuring its separation properties by preferentially interacting with molecules having dipole moments. (C) 2010 Elsevier B.V. All rights reserved.