Core-shell polymer microspheres have been prepared by a two-step precipitation polymerization. These microspheres consist of 3.2 µm polydivinylbenzene-55 (DVB55) cores coated with ∼0.4-0.9 µm wide shells composed of poly(DVB55-co-EDMA), a random copolymer of DVB55 and ethylene glycol dimethyl acrylate (EDMA). The chemical composition, core-shell morphology, and porosity of these structured microspheres have been studied quantitatively with scanning transmission X-ray microscopy (STXM) at a spatial resolution of 100 nm. Images recorded at selected energies through the C 1s absorption region were analyzed using singular value decomposition to obtain quantitative maps of the DVB55 and EDMA components. The EDMA concentration in the shell determined by STXM was found to be in good agreement with that predicted from the comonomer composition over the range of 10-90% EDMA. The precision of chemical quantification at high spatial resolution is shown to be adequate for STXM to be useful in guiding the development of structured polymeric systems for particular applications.