A model for a chemically associating fluid, adsorbed in a disordered porous media, is proposed. The formation of the associates occurs through the directional bonding between the fluid particles. For simplicity, we restrict our attention to the dimerization of particles. In the absence of association, this model reduces to that of Kaminsky and Monson (KM) for the adsorption of methane in a xerosilica gel. This model is studied by means of the replica Ornstein-Zernike ROZ equations, with the hypernetted chain approximation, extended for associating fluids. It follows from a comparison with the computer simulation data that this theory yields a very good description of the structural properties of the KM model. The influence of the fluid density, the matrix packing fraction, and the association energy on the dimerization in the disordered matrix is studied. The fluid compressibility for the KM model and for the dimerizing fluid in a disordered matrix is obtained via the compressibility equation.