We have considered a simple, hard-core-type model for an inhomogeneous system consisting of a dimerizing fluid and a microporous solid membrane with a slit shape, The system is confined to a wide slitlike pore, The bulk and porous regions are in thermodynamic equilibrium. Monte Carlo canonical simulation results are presented for the density distributions of the particles in the bulk fluid region, in layers close to the outer surfaces of the porous solid and inside the crystalline membrane, We have observed that dimers prefer to be located in the interstices of the membrane channels rather than in the membrane cavities. The pressure is obtained from the density profiles on the walls of a wide pore. We show that the adsorption isotherms and the partition coefficients are sensitive to the association energy between fluid particles. For almost all densities, the partition coefficient of the monomers is found to be higher than for those particles that belong to dimers. This reflects the shape selectivity of the membrane with respect to the fluid species during adsorption.