A high-pressure Sieverts apparatus, specifically designed to investigate carbon and other low-density materials for hydrogen storage, has been constructed and used to investigate potential storage materials for which the volume of the sample is uncertain or difficult to define. The apparatus can be managed from a computer via a graphical interface and used to measure gas sorption isotherms from 77 K to 873 K, utilising a computer driven piston pressure booster to compress the gas to 340 bar. Based on measurements of activated carbons and graphene-like samples, this article demonstrates the low sensitivity of the apparatus to uncertainty in the sample volume and the quality of data obtained for H-2 adsorption isotherms measured at 296 K and 77 K at pressures up to 300 bar. Crown Copyright (C) 2017 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. All rights reserved.