The effect of a low energy air-plasma treatment of extended chain polyethylene (ECPE) fibres (Spectra(R)900) on the adhesion with a matrix of epoxy resin has been studied. Surface energies of fibre and matrix were calculated by contact angles, measured with a Wilhelmy microbalance in different liquids, and the adhesion between fibres and the matrix was evaluated through a pull-out test. The results showed an increase in fibre-matrix adhesion by a factor of ca. 1.5 calculated by surface energy measurements, and by a factor of ca. 4 measured by the pull-out tests. Time (up to six months) and temperature (in the range 20-120-degrees-C for 2 h) ageing caused some decrease in adhesion with respect to the values evaluated just after the fibre plasma treatment. The plasma treatment did not affect the fibre’s mechanical properties.