Peel force measurements as a function of adherend thickness are reported for adhesively bonded specimens based on a cyanate ester resin and aluminium adherends. It has been demonstrated that by incorporating large diameter (0.28 mm) PTFE monofilament within the adhesive bond then the peel force and associated fracture energy can be increased significantly over that for specimens based on adhesive alone. Fracture energy measurements are derived for specimens with peeling adherend thickness of up to about 0.6 mm using the 90 degrees peel test. Fracture energies are also derived for peeling of more practically-representative 1.6 mm thickness adherends using a single cantilever beam experiment. In-situ photoelasticity and SEM microextensometry experiments are reported which show the stress fields and displacements associated with the presence of the monofilament. It is believed that the reported increase in measured fracture energy is partly due to the crack pinning effect of the monofilament, and partly due to the monofilament creating a "load shadowed" region between:adherend and monofilament which prevents the interfacial crack from propagating between adherend and adhesive.