The effect of different pH and addition of ferulic acid, tannin, cornstarch and hydrogen peroxide into the film forming solutions on in vitro protein digestibility, content of available lysine, and mechanical and water vapor barrier properties of films made of soy protein isolate were investigated in our experiment. The results showed that protein digestibility and content of available lysine decreased after film formation. Moderately higher pH, addition of cornstarch, ferulic acid and tannin improved mechanical and water barrier properties. However, except for the addition of cornstarch, these substances further decreased protein digestibility and the content of available lysine compared with the same pH treatment. Addition of hydrogen peroxide decreased protein digestibility but did not greatly improve the mechanical and water vapor barrier properties of film. It seemed that a full estimation of the nutritional properties of protein based edible films is necessary before they launch into industrial application as edible films rather than as general packaging materials. (C) 2003 Elsevier Ltd. All rights reserved.