In this work, the membrane separation of hydrogen from binary, ternary and quaternary mixtures of H-2, N-2, CO and CO2 is presented. Hydrogen permeability through a polyimide Matrimid 5218 membrane was experimentally obtained using the constant pressure technique. The influence of the feed gas composition, temperature (30-100 degrees C), pressure range (up to 6 bar), and flow rates was experimentally analyzed. As expected, the pure gas permeability of H-2 was only slightly dependant on pressure and had an average value of 17.7 x 10(-14) m(3)(STP)m m(-2) s(-1) kPa(-1) at 30 degrees C. Hydrogen permeability was not affected by the presence of nitrogen and carbon monoxide, and as a result the mixed gas selectivities for the H-2/N-2/CO mixtures are very close to the selectivities calculated from pure gas permeation data. On the contrary, a strong dependency of the hydrogen permeability on CO2 concentration was observed even at low concentrations of CO2. A reduction of 42% of the hydrogen permeability coefficient was obtained when a mixture of 10/90 vol.% H-2/CO2 was used as feed gas. Accordingly H-2/CO2 selectivity decayed from a value of 4.2 calculated from pure gas permeabilities to 2.7 when permeation data were obtained in mixed gas experiments. The preferential sorption of CO2 on the Langmuir sites of the excess free volume portion of the polymer allowed explaining and quantifying this phenomenon. The "dual-mode sorption, partial immobilization" model was used to describe H-2 and CO2 permeation behavior of pure, binary, ternary and quaternary mixtures. The model sorption parameters for N-2 and CO2 in the polymer Matrimid 5218 were obtained from the literature meanwhile those for H-2 and for CO were unknown and resulted from the fitting of the experimental data to the proposed model. Satisfactory agreement between predicted permeability results and experimental data with a correlation coefficient (R) higher than 0.95 and mean squared relative error (MSRE) lower than 0.01 was attained. Thus, this work reports useful knowledge related to the intrinsic material properties, considering gas mixtures of industrial interest and essential when other membrane configurations like hollow fibers, mixed matrix membranes or polymer blends are proposed. (C) 2011 Elsevier B.V. All rights reserved.