Soy protein-based films modified with gelatin and plasticized with glycerol were prepared by compression at pH 10. The effect of different contents of lactic acid, epoxydized soybean oil and olive oil on optical, barrier, and mechanical properties was investigated and results were related to changes in the bands obtained by Fourier infrared spectroscopy and thermo-gravimetric analysis. The observed changes indicated interactions of the small molecules of lactic acid with protein and glycerol, which also caused the improvement in the hydrophobic character of the films, maintaining water vapour permeability and good values for puncture and UV barrier properties and, at the same time, decreasing the typical yellowish colour of soy protein based films. (C) 2011 Elsevier Ltd. All rights reserved.