The plastic deformation mechanisms of isotropic and blown High-Density Polyethylene (HDPE) and Linear-Low Density Polyethylene (LLDPE) films were studied. The deformation process was followed in stages using dichroic FTIR measurements. Our results indicate that the mean orientation direction of the crystalline regions determines whether or not the polyethylene sample exhibits double yielding behavior in a particular stretching direction, regardless of its crystalline content.