Post-yield, heterogeneous deformation in the form of dilatational bands (DB's) develops in blown, linear low density polyethylene films under certain biaxial loading conditions. The material within the dilatational bands is drawn transverse to the initial orientation direction. The films typically exhibit a two-stage drawing process associated with migration of the DB boundary into the undrawn material and subsequent deformation of the drawn material within the DB. Kinematic measurements reveal that DB's evolve primarily by isotropic expansion with distortion occurring to a smaller extent. Correspondingly, the M integral is used to determine the energy release rate associated with isotropic expansion of the DB under various biaxial stress states. A thermodynamic model incorporating the M integral is used to determine a material property that describes the energy associated with the drawing process. The M integral and the thermodynamic model appear to describe DB evolution well. (C) 2000 Published by Elsevier Science Ltd.