Recognising the difficulties encountered in measuring the adsorption of hydrogen at high pressure, a volumetric differential pressure method has been developed to improve the accuracy of the determination of the hydrogen storage capacity of carbon materials at pressures of ca. 10 MPa. The apparatus used has two identical limbs, a sample limb and a blank limb, between which a high accuracy differential pressure cell measures changes in pressure. By expanding simultaneously the contents of the two limbs and closely controlling the temperature of the entire system, many of the errors due to expansion of the gas can be avoided. In addition, helium blank measurements are used to give a base line correction, which corrects the cooling effect caused by the rapid expansion of gas through a small port. Consequently, many of the errors that have been suggested as causes of some of the inconsistencies that have arisen in this field can be avoided. Using this method, the hydrogen storage capacity of small samples of carbon materials (1-2.5 g) can be measured accurately to 0.1 wt%. Examples of the hydrogen adsorption capacity of some carbon materials will be given and potential barriers for this technology explored.