We have studied the competitive adsorption of binary mixtures of diblock copolymers, differing in molecular sizes of the two blocks, from solution in toluene onto mica and oxidized silicon substrates. We have considered three systems in which the two molecules are both poly(2-vinylpyridine) (PVP)-polystyrene (PS) diblock copolymers and one case where one of the molecules is a PVP-PS diblock copolymer and the other molecule is a poly(2-vinylpyridine)-polyisoprene (PI) diblock copolymer. Toluene is a selective solvent for the block. copolymers, poor for PVP and good for both PS and PI. We find that in the case where there are small differences in the PVP block sizes and large differences in the PS block sizes, the adsorption favors the molecule with the smaller PS block size. In the case where there is a small difference in the PS block. sizes and a large difference in the PVP block sizes, the adsorption favors the molecule with the larger PVP block. In the case where there is a difference in the chemical species, the contour length of the nonadsorbing block is the appropriate measure of size for comparing molecules to determine which molecule will be favored.