In tungsten plasma-facing fusion reactor components, Ta is the third most abundant element formed by transmutation (after Re and Os), yet little is known about the behaviour of W-Ta alloys under irradiation and any effects Ta might have on Re clustering in W-Re-Ta alloys. In this study, W-4.5 at.%Ta, W-2 at.% Re-1 at.%Ta andW-2 at.%Re alloys were exposed to 2 MeVWþ ions to a fluence of 2.64 x 1015 ions cm-2 at temperatures of 573 and 773 K. Atom probe tomography and nanoindentation were used to characterise the chemical and physical properties of the irradiation-induced clusters and the mechanical properties of the irradiated layer. In the W-Ta alloy, no evidence of irradiation-induced clustering was found. In theW-Re-Ta alloy, at both irradiation temperatures studied the presence of Ta reduced theWRe cluster number density and volume fraction compared to that in the W-Re alloy; however it did not alter cluster composition as the Ta was rejected from the clusters. The reduced cluster density in the WRe- Ta alloys was associated with a smaller degree of irradiation hardening than in the W-Re alloys. Possible mechanisms by which Ta hinders cluster development are discussed.