Polyethylene terephthalate microfibril-reinforced composites with linear low-density polyethylene (LLDPE) matrix were manufactured by four industrial processing methods: compression moulding, slit-die extrusion with calendering, extrusion blow moulding and injection moulding. The microfibrils were generated in situ by blending the two polymers together and thermomechanically processing the blend. Thin sheets were manufactured in all the processes, except injection moulding, which was employed for making containers. The oxygen permeability of the films was determined using a commercial machine. The greatest improvement in oxygen barrier property (compared to neat LLDPE films), was found in the compression-moulded microfibrillar composite (MFC) films. Two existing theoretical models were modified to reasonably forecast the oxygen permeability of compression-moulded MFC films. The parallel filler model provides a very good fit to the experimental data for the full mass ratio range. The Lewis and Nielsen model was also trialled for various polymer combinations, producing reasonable results with minor modifications.