Neutron reflectivity is used to measure the volume fraction profile of carboxylic acid-terminated polystyrene (PSCOOH) absorbed on silicon oxide from deuterated cyclohexane, a near-Θ solvent. Because of the affinity of the acid group for the silicon oxide surface, the PSCOOH adsorbed amount is 4 times larger than the unfunctionalized PS case at 23 °C. Although both PS and PSCOOH have similar surface volume fractions (ca. 0.60), the PSCOOH layer thickness is 4 times larger, suggesting that the PSCOOH chains stretch by 4 times their unperturbed size. Upon increasing the degree of polymerization N from 67 to 780, the PSCOOH layer thickness scales as N1 under near-0 solvent conditions. Under good solvent conditions, the PSCOOH surface volume fraction decreases by a factor of 3, whereas the layer thickness remains the same as in the near-Θ case. In both solvents, the reflectivity can be fit with PSCOOH profiles which are parabolic with some rounding near the free chain ends. Neutron reflectivity studies of DPSCOOH and PSCOOH adsorbed from cyclohexane and cyclohexane-d, respectively, suggest that DPSCOOH adsorption is slightly stronger due to isotope effects. © 1995, American Chemical Society. All rights reserved.