A volumetric apparatus for hydrogen adsorption measurement aimed at vacuum system was constructed. The performance of the apparatus was assessed by using the composite adsorbent that includes molecular sieve 5A and differential proportion of PdO and Ag(2)O. In order to avoid the accumulative error, the whole system is evacuated every time for each point of pressure, instead of changing pressure step-by-step and summing up the amount of adsorption at each step. Iterative calculation of processing experimental data is used. Experimental results and theoretical analysis showed that the volumetric apparatus, the procedure and the method of data processing described in this paper can provide an accurate measurement of hydrogen adsorption in vacuum system. The capacities of freshly activated composite adsorbent for hydrogen are found to increase markedly at low pressure after consecutive adsorption-desorption cycles. The phenomenon is known as the so-called "conditioning effect". Besides the experimental errors, the large discrepancies on the reported adsorptive capacities of hydrogen on various adsorbents may be due to incomplete conditioning of the adsorbent, composition of the adsorbent and the slow rate of approach to equilibrium. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.