Moisture sorption isotherms ( a w=0.115–0.94) were measured for a series of whey protein isolate (WPI) edible films prepared with different amounts of glycerol as a plasticizer (glycerol:total non-volatile material G=0–0.5) and for native WPI. The moisture sorption characteristics of the native protein were similar to those of the G=0 films and the equilibrium moisture content of the films increased linearly with G at all humidities. The Guggenheim-Anderson-de Boer was superior to either the Peleg or BET model as a model for the measured isotherms. The equilibrium moisture content of the films, M, was modeled using a four-parameter empirical model, M=k 1 exp (k 2a w )+k 3 exp (k 4a w )G , where k 1–4 are 0.0016, 2.72, 0.0046, and 6.24, respectively ( r=0.98).