This paper presents a constitutive model capable of predicting the thermoviscoelastic behavior of the balloon thin film StratoFilm subject to large strains up to yielding. The model is based on the free volume theory of nonlinear thermoviscoelasticity and extended to orthotropic membranes. An ingredient of the present approach is that the experimentally inaccessible out-of-plane material properties are determined by fitting the model predictions to the measured non-linear behavior of the film. Creep tests, uniaxial tension tests, and biaxial bubble tests are used to determine the material parameters. The model has been validated experimentally, against data obtained from uniaxial tension tests and biaxial cylindrical tests at a wide range of temperatures and strain rates spanning two orders of magnitude (0.01%/s∼1%/s). © 2015, American Institute of Aeronautics and Astronautics. All Rights Reserved.