A saturation of the yield strength appears in the post irradiation hardening of copper irradiated with high energy heavy ions, in the range from 10(-3)-10(-2) dpa. The saturation value of the yield strength is significantly lower than that measured after irradiation with 14 MeV neutrons to comparable doses. To investigate the origin of this difference in behaviour, a detailed transmission electron microscope study of the post-irradiation defect cluster microstructure was undertaken. In specimens irradiated with 440 MeV Ar and 230 MeV ions up to 0.01 dpa the microstructure is composed by large dislocation loops (up to 25 nm), stacking fault tetrahedra (3 nm) and small defect clusters (< 2 nm). In the case of the Ar irradiation, the total defect density is 2.8 x 10(23) m(-3) at 10(-2) dpa. Of these, 40-50% are stacking fault tetrahedra, whereas only 1-5% are large loops. The total defect cluster concentration shows a (dose)(0.5) behaviour, with no saturation up to highest dose observed (10(-2) dpa).