This research uses spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to determine an upper bound for the interfacial width of a solution-cast poly(styrene) (PS)-poly(2-vinylpyridine) (PVP) homopolymer blend. The measurement determines the fraction of nitrogen as a function of position across an unstained interface. The broadening effect of the incident-probe intensity distribution is deconvoluted from the raw data. In addition, a lower bound to the contribution of interfacial curvature to the interfacial width is estimated and also separated from the measured data. This leads to an upper bound to the interfacial width of 3.5 nm. The result is in agreement with independent measurements by neutron scattering reported in the literature. Dose-resolved measurements are made to demonstrate that the effects of mass loss during the present measurements are insignificant. Quantitative analysis of the carbon/nitrogen ratio is made on the basis of the background-subtracted C and N K-edges in PVP finding a value of 7.05 +/-0.20, in agreement with the stoichiometric value of 7.0.