A model for the adsorption of dimers at a surface is presented. The model is based on a generalization of the Ono-Kondo lattice theory, The densities of dimer molecules that are parallel to the surface, x(parallel to), and perpendicular to the surface, x(perpendicular to), are calculated. It is shown that symmetric dimer molecules adsorb preferentially parallel to the surface at all densities. The model also predicts that the Gibbs adsorption is negative for small interaction energies between dimers and the surface.