Resonant soft X-ray scattering (RSOXS) is a complementary tool to existing reciprocal space methods, such as grazing-incidence small-angle X-ray scattering, for studying order formation in polymer thin films. In particular, RSOXS can exploit differences in absorption between multiple phases by tuning the X-ray energy to one or more resonance peaks of organic materials containing carbon, oxygen, nitrogen, or other atoms. Here, we have examined the structural evolution in poly(3-hexylthiophene-2,5-diyl)/[6,6]-phenyl-C61-butyric acid methyl ester mixtures by tuning X-rays to resonant absorption energies of carbon and oxygen. Our studies reveal that the energy dependence of RSOXS profiles marks the formation of multiple phases in the active layer of organic solar cells, which is consistent with elemental maps obtained through energy-filtered transmission electron microscopy. © 2013 American Chemical Society.