Calcium-carbonate-filled linear low-density polyethylene (LLDPE) films play an important role in the hygienics market because these materials allow for the diffusion of water vapor (breathability) and retain a liquid barrier while providing a barrier to liquids. This is achieved when properly formulated composites are stretched to create pores. There are many important issues surrounding this technology; this report focuses on the effects of poststretching heat-set treatment on moisture vapor transmission rate (MVTR), dynamical mechanical thermal analysis (DMTA), tensile heat distortion temperature (THDT), and differential scanning calorimetry (DSC) thermal transitions of the porous film properties. Eastman personnel provided the LLDPE/CaCO3 breathable films for this study. In general, the film properties were insensitive to heat-setting time beyond that of 1 min. This result suggests that the molecular reorientation and recrystallization associated with the changes in film properties occur rapidly. Properties were, however, strongly sensitive to the heat-set temperature when MVTR decreased and DMTA properties [specifically storage modulus (E ') and the alpha -transition temperature] increased as the heat-set temperature increased. This is believed to have been mainly caused by pore closure and the annealing of crystalline regions. The heat-set temperature was easily detectable by DSC techniques when a thermal transition was apparent at the applied temperature, and changes in the heat of fusion for the sample could be observed. Ultimately, the changes in the properties of these porous films were related to changes in the molecular orientation and crystallinity of the matrix in combination with changes in the void structure of the composite, In particular, changes in E ' could be related to these separate effects.