The influence of the oxidized carbon fibre content on the properties of thermoplastic elastomeric matrix (styrene-butadiene, SBS) has been studied, in comparison with those obtained in commercial carbon fibre composites at the same fibre levels, Tensile strength, elongation at break and tear resistance variations place the optimum fibre content at 10-15 phr. The stress-strain behaviour points to the differences between both types of composites which are explained in terms of the existence of a strong interface matrix oxidized fibre in the composite, Although the fibre addition negatively affects the stress relaxation behaviour, at the same fibre content the stress relaxation is slower in oxidized fibre composites.