Monte Carlo simulation is used to study the adsorption of diblock copolymers and homopolymers on bare surfaces and surfaces modified by end-grafted polymers at densities well below the brush limit. Monomer distribution profiles for the adsorbed polymer are used to compute the surface excess (Gamma(exc)). The competitive adsorption of asymmetric diblock polymers on bare surfaces is studied first, in order to compare with experimental results and to test the validity of our method. Gamma(exc) values can be used to obtain the adsorption isotherm, which are fit to a Langmuir model in order to estimate the maximum surface excess, Gamma(exc)(max), as a function of end-grafted polymer coverage and molecular weight. Competitive adsorption of different molecular weight polymers is simulated and the relative adsorbed amounts correlate well with Gamma(exc)(max) for the individual polymer. We find that the adsorption of symmetric diblock polymers is very similar to that of homopolymers. Surfaces modified by end-grafting mixtures of two different lengths of polymers are also considered. Bimodal surface modification creates density profiles that differ strongly from monodisperse end-grafts, and in some cases this serves to enhance the effect of selective size adsorption.