The interphase region in dry and water aged polymer/glass fibre composite materials was investigated by means of the nano-indentation and the nano-scratch techniques. The indentations as small as 30 nm in depth were produced along 14 μm path starting from the matrix and ending on the fibre. This was done in order to detect water degradation of material properties in the transition region between the fibre and the matrix. The distinct properties of the interphase region were revealed by a sequence of two to three indents in dry materials and up to 15 indents in water aged, degraded materials. These results indicated interdiffusion of water into the interphase regions. The nano-scratch test results showed that the interphase region width increased and the material properties degraded during water aging. Doubts about the presence of the interphase, raised over the hardness results in the vicinity of the harder glass region, were refuted with the results showing the expansion of the interphase region during water aging. These two experimental techniques showed to be a useful tool in the investigation of the size and the character of the interphase region in the fibre/polymer matrix composite materials. © 2001 Elsevier Science Ltd.