Carbon based adsorbents, especially activated carbons are one of the most widely used adsorbents. This is attributed to its excellent adsorption capacity as well as its better stability in the presence of moisture and corrosive gases. However the pores in these activated carbons in general, are often ill-defined, irregular and irreproducible. Modeling the actual pore system in activated carbon is far too complex and this seriously hinders a deeper understanding of the adsorption phenomenon in these materials. Carbon nanotubes (CNTs), on the other hand, have a defined, uniform and a reproducible pore structure. Compared with randomly oriented CNTs, a better well-defined structure is obtained by aligning the carbon nanotubes in a vertical manner. The vertically alignment of carbon nanotubes leads to the creation of multiple well-defined adsorption sites and results in an enhanced adsorption capacity. Vertically aligned carbon nanotubes (VACNTs) can thus serve as an ideal model structure for understanding gas adsorption in carbon materials. In this work VACNTs are synthesized by water assisted chemical vapor deposition technique. The as-prepared CNTs are usually double to multi-walled with an average internal diameter of 8 nm and have a specific surface area of 500 m2/g. To assess the adsorption characteristics of these VACNTs, two different experimental adsorption setups are constructed. A commercially available TG setup is modified to measure the adsorption capacity at ambient pressures. To determine the adsorption characteristics over a wider temperature and pressure range, a high pressure volumetric setup is designed, built, calibrated and installed as a fully operable high pressure device which can handle smaller sample quantities (< 100 mg) and can determine adsorption up to a pressure of 100 bar in the temperature range 10–50 ˚C. In addition, the volumetric setup is designed to measure adsorption isotherms of gas mixtures as well. Using these two techniques, the CO2 adsorption characteristics of the as-prepared VACNTs are determined and the influence of various other parameters like diameter, intertube distance etc. are explored systematically. Functionalization of VACNTs is carried out by preserving the alignment and the effect of addition of various functional groups on gas adsorption characteristics are investigated. Finally the adsorption characteristics of VACNTs are compared with other carbon based adsorbents. The adsorption capacity of VACNTs is found to increase linearly with pressure and at high pressure, the adsorption characteristics of modified VACNTs is found to be superior to that of other standard adsorbents.