Partially deuterated polymers, made by saturating the double bonds of polydienes with deuterium, have been found to produce weak but significant coherent patterns in small-angle neutron scattering (SANS) experiments : The results suggest the presence of slight differences in deuteration levels among the chains. The scattering profiles for such materials and for blends containing them were calculated with the multicomponent random phase approximation (RPA). The coherent intensity for the deuterated component alone is predicted to be proportional to the variance of its Scattering length distribution and to have a q dependence corresponding to the pure component structure factor. The SANS data for a variety of partially deuterated polyolefins are shown to be consistent with these predictions. The theory also predicts that the scattering from their blends with an hydrogenous component is enhanced by this scattering length inhomogeneity, that the enhancement is additive, and that the conventional two component RPA may be used to analyze the data after this contribution is subtracted. Experimentally, we find that uncorrected scattering from blends rich in such deuterated components lead to anomalous estimates of chain dimensions and thermodynamic interactions. The anomalies disappear when the proposed correction is applied.