Physical properties and microstructure of films made of mixtures of whey protein isolate (WPI) and mesquite gum (MG) were investigated. A photoacoustic method was used to measure water vapor diffusivity. WPI and MG were totally compatible to make solutions and films. Results showed that film structure is based on a continuous network of WPI aggregates which enlarge their size as the MG content increases. Neither water vapor diffusivity nor permeability was affected significantly by MG content. However, the incorporation of increasing amounts of MG in the formulation resulted in films with improved flexibility, that is, with significantly lower tensile strength and higher elongation at break. This result suggests the use of MG to improve mechanical properties of WPI films as an alternative of using larger amounts of low molecular weight plasticizers. (C) 2008 Elsevier Ltd. All rights reserved.