The properties of the interphase formed between a glass fibre and a polymer resin have been characterised based on novel techniques, including the nanoindentation and nanoscratch tests and the thermal capacity jump measurement. The variation of interphase thickness affected by differing silane coupling agents is specifically evaluated. The nanoindentation test gives an interphase thickness of approximately 1 μm with large variations between specimens, and is not sensitive enough to identify the effect of different silane agents. The effective interphase thickness measured from the nanoscratch test varies between 0.8 and 1.5 μm depending on the type and concentration of silane agent. The higher is the silane agent concentration, the larger is the interphase thickness. These values are consistent with those measured based on the heat capacity changes in terms of general trend, although the latter technique tends to present slightly higher values. The foregoing observations strongly support the usefulness of the techniques for interphase characterisation.