When two polymers are brought into intimate contact, the interface is not perfectly sharp. Instead, localized molecular mixing produces an interphase with thickness on the order of 10 nm. Forced assembly by layer multiplying coextrusion makes it possible to create films that are entirely interphase. The new interphase materials can be characterized using conventional tools of polymer analysis. In this study we vary the ratio of poly(methyl methacrylate) (PMMA) to polycarbonate (PC) in the coextrusion process in order to obtain very thin PMMA nanolayers sandwiched between thicker (similar to 50 nm) PC layers. We use oxygen permeability to probe the nature of the interphase as the PMMA layer thickness is reduced below the interphase thickness to the limit of layer stability. If the PMMA nanolayers are made thinner than the 12 nm interphase dimension, they more closely resemble a thin interphase region sandwiched between thicker PC layers than they do discrete PMMA layers. The interphase region becomes progressively thinner with decreasing PMMA layer thickness until the limit in melt stability of the PMMA layer is reached. Efforts to obtain PMMA nanolayers thinner than 5 nm resulted in layer instability and breakup. Oxygen permeability suggested that the nanolayer fragments had a very high aspect ratio.