Thin films of various thicknesses ranging from 100 mu m down to 1390 Angstrom of an isotopic polystyrene blend were examined using small-angle neutron scattering. It was found that absolute scattering data with reasonable signal-to-noise could be obtained from single films as thin as 1390 Angstrom supported on single-crystal Si wafers by optimizing the neutron optics and by carefully eliminating background arising from the substrate. The wave vector dependence of the neutron scattering from all of the films examined could be described using the random phase approximation with the same Flory-Huggins interaction parameter and R-G values as found in the bulk. No dependence of the scattering on film thickness was found. These results demonstrate that quantitative small-angle neutron scattering measurements can be obtained from single ultrathin films of polymer mixtures.