Twaron aramid fibers require a surface treatment process in order to optimize their adhesion to polymer matrix systems.In this process, a finish containing epoxy reaction products is applied to the fiber surface and subsequently heat treated.The resulting fiber surface yields improved interfacial adhesion to, for example, epoxy and rubber matrixes.In order to assess the role of the chem. and topog. modified fiber surface, detailed anal. of the interphase is a prerequisite.Well-defined starting materials and model components combined with surface anal. techniques are used.The reaction products in the epoxy-based finish are determined using a model formulation.The reaction chem., referred to as 'maturation' of the finish, and its relationship to an important process parameter, the pH of the formulation, is established.Quant. information on the dimensions and the chem. composition of the interphase, the presence of functional groups at the fiber surface and the correlation between the interphase structure and the measured adhesion level to an epoxy matrix is studied.