Fabrication of a breathable film by the microlayer coextrusion process is described. Poly(ethylene oxide) (PEO) was microlayered with a filled polyolefin, either CaCO3-filled polyethylene or CaCO3-filled polypropylene. The thickness of individual layers was varied by increasing the total number of layers in the microlayered film from 8 to 4096. The water vapor transport rate (WVTR) was measured for microlayer films that varied in composition and number of layers. Especially with the PP(CaCO3)/PEO system, systematic variation in composition and number of layers made it possible to obtain large changes in the WVTR. The results were related to the tortuousity of the pathway through the microlayer. The filled polyolefins acted as a barrier to water vapor transport through the hydrophilic PEG. As the individual layers were made thinner by increasing the total number of layers, the polyolefin layers changed from continuous to discontinuous. Tortuousity concepts were used to correlate the increase in WVTR with an effective aspect ratio of the discontinuous polyolefin layers. In addition to high WVTR values, the breathable films produced by microlayering PEO with a filled polyolefin exhibited excellent mechanical properties.