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Grain protein films plasticized with glycerol were prepared from corn zein (CZ), wheat gluten (WG), and a 2.3:1 mixture of wheat gluten and soy protein isolate (WG/SPI). Moisture adsorption curves of the three types of protein films at 25° C and within an approximate water activity range of 0.11 to 0.84 were obtained using a static gravimetric method. The Smith, Oswin, Halsey, and Guggenheim-Anderson-de Boer (GAB) models were applied to collected data. For all protein films, the GAB model showed the best fit over the entire studied water activity range yielding mean deviation modulus values (P) of 4.69, 3.44, and 7.95 for WG, WG/SPI, and CZ films, respectively. The Smith and Halsey models fitted well the high water activity (0.53 to 0.84) portion of the isotherms with P values ranging between 2.55 and 6.74. Moisture adsorption behavior by the protein films at the low water activity range (0.11 to 0.58) was reasonably well described by the Oswin model (P values of 6.68,5.25, and 5.24 for WG, WG/SPI, and CZ films, respectively).