Specular X-ray reflectivity from SiO2 thin films prepared on silicon substrates by plasma-enhanced chemical vapor deposition showed the films to have a characteristic width of the decay in density at the free surface of 17 Angstrom, to be about three-quarters the density of alpha-quartz, and to have an interfacial layer at the silicon interface that was of the order of 100 Angstrom wide and less dense than the bulk of the film. After chemical-mechanical polishing the characteristic width of the decay in density at the free surface was reduced to 10 Angstrom; furthermore, the near-surface region to a depth of 30 Angstrom had a greater density than the as-deposited film. Off-specular reflectivity confirmed that the decrease in characteristic width at the free surface was due to reduced roughness upon polishing and also revealed that the lateral correlation length in the limit of long wavelengths was the same for both polished and unpolished samples. The compression of the near-surface region during polishing is believed to enhance the dissolution of SiO2 into the slurry which is necessary to achieve smooth surfaces.