The effects of Hf, Mo and W on the microstructure and hardness of as cast and heat treated (1500 °C/100 h) Nb–20Si–5Hf–5W (YG5), Nb-20Si–5Mo–3W (YG6) and Nb–20Si–5Hf–5Mo–3W (YG8) alloys were studied. The macrosegregation of Si was strong in the as cast alloy YG6 and decreased in the order of the synergy of alloying elements as follows (Mo + W), (Hf + W) and (Hf + Mo + W). All three alloys were contaminated by oxygen. The phases present in the as cast alloys YG5, YG6 and YG8 were the Nb ss , Nb 5 Si 3 and HfO 2 , Nb ss , Nb 3 Si, and Nb 5 Si 3 and Nb ss , Nb 5 Si 3 and HfO 2 respectively, with a lamellar Nb ss + Nb 5 Si 3 microstructure formed in all three alloys. In the as cast alloys YG5 and YG6 there were Nb ss grains with no Si content. There was microsegregation of Hf and W in the Nb ss in the as cast alloy YG8 and microsegregation of Hf in Nb 5 Si 3 in the as cast and heat treated alloys YG5 and YG8. It is concluded that the concentrations of Si and refractory metal (RM) in the Nb ss depend on the synergy of RMs with/without Hf. The Nb ss with no Si and Nb 5 Si 3 were present in all three heat treated alloys and HfO 2 was formed in the heat treated alloys YG5 and YG8 via the consumption of the Hf in the Nb ss . The stability of the Nb ss + Nb 5 Si 3 lamellar microstructure decreased in the order of the heat treated alloys YG5, YG8 and YG6. In the presence of Hf the transformation of βNb 5 Si 3 to αNb 5 Si 3 was enhanced. It is concluded that the equilibrium microstructures of the alloys consist of the Nb ss with no Si and αNb 5 Si 3 phases. Alloying with Hf caused reduction of the hardness of the alloys YG5 and YG8 after heat treatment.