To understand the effect of processing and co-monomer content on interfacial adhesion, we quantified adhesion levels of bilayers of a polyethylene (PE) with various polypropylenes (PPs) prepared using bilayer co-extrusion and lamination processes. We tested adhesion between a medium-density PE (MDPE) with different types of PPs, including impact-modified PP (with various amount of ethylene), isotactic PP and ethylene-propylene random copolymers. Increasing the concentration of ethylene or ethylene-propylene rubber gave rise to increased adhesion. The impact-modified PP with 20 wt% ethylene content exhibited adhesion with MDPE almost two orders of magnitude higher compared with other PPs. Although lamination and co-extrusion processes showed good agreement in these trends with ethylene content, the operation parameters are critical for adhesion control. For lamination, ice-water cooling generated a stronger adhesion than that with air cooling. Faster cooling rates in co-extrusion also gave rise to stronger adhesion. Increasing draw down ratio and varying flow rate to put the interface near the wall resulted in stronger adhesion. Fast quenching rate and increased crystallinity induced by drawing down are believed to be the causes. Both atomic force microscopy and transmission electron microscopic images exhibited roughened interfaces for samples with strong adhesion.