In Enhanced Coal Bed Methane (ECBM) usually CO2 mixtures containing impurities such as NOx, etc., are injected in the coal layer. This paper investigates the adequacy of different Equations of State (EoS) for the determination of adsorption of gas mixtures in a manometric set up. We investigate the viability of these EoS by performing experiments in a reference cell and an empty sample cell at a constant temperature of 318.15 K. From these measurements we derive the volume ratio of these cells. We used a variety of EoS and mixing rules to validate whether they can be applied, i.e., lead to the correct volume ratio. It is shown that for a He rich mixture (98% He. 1% O-2 and 1% NO2) it is possible to use the McCarty EoS for pure helium for this purpose but that none of the other EoS, including mixing rules, give acceptable results. For a CO2 rich mixture (97% CO2, 1% He, 1% O-2 and 1% NO2) none of the investigated EoS and mixing rules can be applied. Therefore, we present an experimental stepwise procedure based on the volume ratio measurements, to obtain correct density values with an error between 0.033% for high densities to 0.85% for low densities. (C) 2011 Elsevier B.V. All rights reserved.