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 Beer 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(2)a(w)) + k(3) exp(k(4)a(w))G, where k(1-4) are 0.0016, 2.72, 0.0046, and 6.24, respectively (r = 0.98).