HY2SEPS (Hybrid Hydrogen - Carbon Dioxide Separation Systems) is one of the EU-funded projects directed at the reduction of CO2 emissions [1]. The principal objective of the project is to develop a hybrid membranę - adsorptive H2/CO2 separation techniąue that would form an integral element of the pre-combustion process. One of the tasks of the Institute of Chemical Engineering, Polish Academy of Sciences is to derive simplified mathematical models for the membranę separation of H2/CO2 mixtures. These models will be included in a comprehensive model of the hybrid system. In the present study one of the simplified models developed is discussed, namely that with the coun-tercurrent pług flow of the feed and permeate. It is assumed that the feed may contain N permeating species, an additional inert component may be present in the permeate, the permeation coefficients are independent of the pressure, and the process is isothermal. The system studied is shown in Fig. 1. The feed gas composition is determined using eąuations (1) and (2), and that of the permeate from equations (3)-(5). Boundary conditions are described by eąuations (6) and (7), and the retentate and permeate flow rates by eąuations (8) and (9). A number of simulations were carried out concerning the separation of binary mixtures that may appear following the steam conversion of methane. The separation of C02 from its 50/50 binary mixtures with hydrogen, nitrogen and methane was studied in a ceramic membranę module over 35-90°C, and at a temperaturę of 60°C for C02 concentrations of 10 to 90 mole percent. The calculational results were compared with the experimental data obtained by FORTH/ICE-HT (Greece) [6]. In Tables 1 and 3 the basie parameters of the process analysed are shown; Table 2 summarizes all the cases investigated in the present study. The estimated fluxes of CO2 and H2 are shown alongside those measured experimentally as a function of temperaturę and C02 partial pressure in, respectively, Figs 2 and 3. The results concerning the C02/CH4 and CO2/N2 mixtures are shown, respectively, in Figs 4 and 5 and Figs 6 and 7. It is concluded that, generally, the CO? flux increases monotonically with both temperaturę and CO? partial pressure. It is also found that the fluxes of hydrogen, methane and nitrogen reach a minimum at a temperaturę slightly above 50°C. Overall, a good agreement was obtained between the simulations and experiments, despite using a simplified description of trans-membrane mass transfer and employing permeation coefficients for pure species.