Using low-energy forward recoil spectrometry (LE-FRES) and neutron reflectivity (NR), the interfacial excess, z*, of an asymmetric poly(deuterated styrene-block-methyl methacrylate) (dPS-b-PMMA) at the polymer matrix/silicon oxide interface was found to decrease as the bromostyrene mole fraction, x, in a poly(styrene-ran-4-bromostyrene) (PBrxS) matrix systematically increased. For matrix degrees of polymerization, P = 480 and 3846, z* decreased by 15% and 33%, respectively, as x increased. Neglecting the matrix-substrate interaction energy, epsilon(M)(s), self-consistent mean-field (SCMF) calculations predicted an increase in z* with x, consistent with an increase in unfavorable matrix-dPS interactions, gamma. By including a small attractive interaction (epsilon(M)(s) = -0.01 k(B)T) between the matrix and substrate, the SCMF z* decreased by ca. 50%, in qualitative agreement with experiment. Thus, as x increased (and therefore epsilon(M)(s)), matrix chain competition for silicon oxide counteracts the expected increased adsorption due to chi. Furthermore, the dPS volume fraction profile in PBr0.136S had a lower z* and was thinner than for the neutral matrix case.