The thermal mechanical properties of the interphase in glass fibre/polymer composites were measured by means of the newly developed dynamic single fibre load (DFL) test. The influence of the thickness of a highly viscoelastic polyester-polyurethane fibre coating on the behaviour of a glass/epoxy composite was measured and compared with the results of a computer simulation. This shows, that the information supplied by the DFL test originates from the molecules within a cylinder of about 10 mu m in diameter around the fibre. In the second part of the paper the influence of a commercial fibre coating on a glass/PS composite was investigated. It was shown that due to the fibre coating the glass transition temperature, T-G, shifts to higher temperatures while the activation energy is increased.