Three different unidirectional polymer–glass composite systems involving phenolic and polyester resins were aged for 6 and 11 weeks in tap water and tested in the mode I double cantilever beam (DCB) test. The results showed a dramatic increase in water absorption and a decrease in fracture toughness for phenolic/glass systems. Fractographic analysis revealed interfacial debonding to be dominant failure mechanism, indicating a strong influence of water degradation on fracture toughness results. The interphase region of each system was investigated using the nano-indentation and the nano-scratch techniques before and after aging in water. The nano-indentation test produced a series of indents as small as 30 nm in depth, to detect water degradation of the material properties at the interphase region between the fibre and the matrix. The nano-hardness results indicated interdiffusion in water aged interphase regions. The nano-scratch test was used in conjuction with the nano-indentation test, in order to detect the width of the interphase regions before and after water degradation. It was shown, from the coefficient of friction and the scratch profile depth, that the interphase region width increased and the material properties degraded during water aging. Qualitative links between water degradation of the glass–polymer interphase on a nanometer level and interlaminar fracture toughness are discussed.