The interphase between two immiscible glassy polymers was probed using nanolayer films with tens to thousands of alternating layers of two polymers. Various combinations of poly(methyl methacrylate), polycarbonate, and a series of styrene-acrylonitrile copolymers were brought together by forced assembly. Continuous nanolayers with thickness on the size scale of the interphase were observed directly using atomic force microscopy. Interphase thickness was extracted from the layer thickness dependence of oxygen permeability. The interphase thickness showed the predicted dependence on the χ interaction parameter and correlated with interphase strength as measured with the T-peel test. Interphase specific volume, as determined by density, exhibited both positive and negative deviations from constituent additivity. The deviations correlated with the change in free volume hole size from positron annihilation lifetime spectroscopy but did not correlate directly with the X parameter. The origin of the deviations was found in the nature of chain packing in the interphase, as evidenced by nonadditivity in entanglement molecular weight. The volumetric effects accounted for the glass transition behavior of the interphase material.