In the film blowing process, LLDPE shows a variety of instabilities with increasing production rate. The purpose of this study is directed at understanding tine mechanism underlying the onset and development of the first instability to occur with increasing flow rate. The onset and progress of the surface distortion is studied as a function of process conditions, die geometry and polymer blend composition, using a Betol 20 mm extruder, equipped with an abrupt entry slit die allowing rheo-optical measurements. We show the qualitative characteristics of the surface distortion obtained via scanning electron microscopy and quantitative surface roughness data obtained through stylus surface profile measurements. In a processing window of 160-200 degrees C, the severity of the instability for LLDPE increases with increasing exit stress level Under these conditions pure low density polyethylene (LDPE) does not show the instability, but blends of the two materials show significantly enhanced extrudate distortions versus pure LLDPE. It is also shown that pure LDPE does exhibit the same unstable behaviour at lower temperatures around 140 degrees C. Using flow birefringence measurements the stress fields in the die can be studied in detail, simultaneously to monitoring the extrudate appearance. Polyflow simulations allow further in-depth investigation of the stress concentrations and local velocities of the fluid in the exit region. Large velocity gradients, deformations and stresses are found near the exit, confined to the surface region. The experimental and numerical evidence obtained from this work is evaluated in relation to a variety of likely mechanisms and appears to particularly support a periodic melt rupture mechanism at the die exit.