A model of localized site adsorption of a dimerizing fluid on a crystalline surface of (100) symmetry is investigated. A heterogeneously dimerizing (A + B reversible arrow AB) bulk fluid, with a square well associative interaction, is described using the model of Cummings and Stell and a particle-particle reference interaction site approximation which includes a complete expression of the intramolecular structure factor. The solid atom-fluid particle nonassociative interactions are Lennard-Jones potentials. An associative square well interaction between the fluid particles and the surface atoms is included. The model is studied numerically using singlet level integral equations for density profiles. The adlayer consists of associatively and nonassociatively adsorbed monomers and dimers. Complexes of fluid particles and surfaces atoms, formed at the on-top adsorption site, the 2-fold bridging site, and the 4-fold hollow site positions are studied. The parameters which influence the structure of the adlayer are investigated.