An accurate (and low-cost) experimental technique has been developed to screen the effectiveness of an adsorbent in improving gas storage capacity within a pressurized vessel. Specifically, the capsule technique is shown to be effective in directly measuring the total H-2 contained within a pressurized vessel and can be used to evaluate the amount of gas in the free space and adsorbed on the solid, that is, a sorption isotherm. The capsule technique was benchmarked by measuring isotherm data for CH4 on an activated carbon sample and was then subsequently evaluated for use with H-2. The capsule data are in excellent agreement with the total storage capacities expected from calculations using equation of state information. In addition, H-2 isotherm data from the sorption capsule are found to be within 1% of values obtained from a more sophisticated differential pressure adsorption unit (DPAU). Conditions for when the adsorbent aids or hinders storage are also discussed in terms of the 2010 DOE H-2 storage targets.