Air- and ammonia-plasma treatments markedly improve the adhesion of gel-spun high-modulus polyethylene (PE) tapes to epoxy resin, as measured by pull-out and evidenced by the change in failure mode from interface controlled to shear failure within the PE. By monitoring the changes in wetting, surface topography and surface composition, the factors responsible for the adhesion could be identified. These results suggest that for air-plasma-treated PE tapes the adhesion depends on (1) the physicochemical interaction between the various carbon-oxygen functionalities and the epoxy resin, (2) mechanical interlocking, and (3) non-polar dispersion forces. The pull-out strength is the sum of these three terms and their contributions in this specific system are about 76%, 12% and 12%, respectively. Physicochemical interactions of the amine groups, in addition to non-polar dispersion forces, determine the adhesion of ammonia-plasma-treated PE tapes to epoxy resin. Their contributions are 87% and 13%, respectively.