Recent research at EMPA hasshown that sustained loading and creep cannot solely explain an apparent 10% reduction in the ultimate tensile capacity of pultruded epoxy matrix CFRP tendons taken from the Kleine Emme footbridge in Switzerland after 17.5 years in service in a typical temperate service environment. This paper interrogates the role of slow (i.e. over decades in service) post-curing reactions in the degradation of tensile properties of the CFRP tendons. Physical aging of the matrix was investigated by Dynamic Mechanical Analysis and nano-indentation tests which showed an increase in glass transition temperature and elastic modulus with increasing post-cure of the epoxy matrix. The evolution of post-cure reactions, which may lead to a decrease in capacity of the matrix to efficiently transfer loads between fibers, was found to be sensitive to the temperature history of the CFRP tendons. The cumulative impact of physical aging mechanisms on the structural performance degradation of fibre reinforced polymers highlights the importance of comprehensive long-term material characterization in structural engineering application.