Polymer-reinforced polymer composites have been successfully created from blends of engineering and commodity plastics. These microfibril reinforced composites (MFCs) contain molecularly oriented in-situ fibrils, evenly dispersed throughout a homogeneous matrix. MFCs have several potential applications in a range of areas including food packaging materials and biomedical scaffolds. This paper provides an overview of the characterisation of several MFCs derived from polyethylene/poly(ethylene terephthalate) (PE/PET) blends. It investigates the tensile properties of MFCs with different microfibril orientations as a result of various consolidation techniques. It also describes a study on the oxygen permeability of moulded MFC films and evidence of biological cell growth on microfibrillar polymer structures.