In this study, we compare the formation of radiation induced defects in W and W-Re-Os alloys, exposed to an equivalent dose of self-ion and neutron irradiation. Transmutation reactions in the neutron irradiated material are simulated in the ion implanted materials by alloying with representative quantities of Re and Os (1.4 and 0.1 at.% respectively). Differences in nano hardness, Re/Os clustering and void formation are compared using a combination of nanoindentation, atom probe tomography (APT) and transmission electron microscopy (TEM). Large voids and Re/Os rich clusters are formed during neutron irradiation. Ion irradiation led to the formation of small voids, as well as small diffuse clusters which consist primarily of Os. The Os clusters formed under ion irradiation supress the formation of Re clusters. Irradiation hardening was observed in all the materials but was greatest in the neutron irradiated material in which the defects were the most pronounced.