Microstructures of 76Mo–14Si–10B, 77Mo–12Si–8B–3Al, and 73.4Mo–11.2Si–8.1B–7.3Al alloys, processed by reaction hot pressing of elemental powder mixtures, have shown α-Mo, Mo3Si, and Mo5SiB2 phases. In addition, particles of SiO2 formed from the oxygen content of raw materials could be seen in the 76Mo–14Si–10B alloy, while α-Al2O3 formed in the alloys containing Al. Parts of the Al have been found within the solid solutions of α-Mo and Mo3Si. The average fracture toughness determined from indentation crack lengths and three-point bend testing of single edge notch bend specimens lies in the range of 5.0–8.7 MPa√m, with alloys containing Al demonstrating higher values. Analyses of load-displacement plots, fracture profiles and indentation crack paths have shown evidence of R-curve type behaviour and operating toughening mechanisms involving crack bridging by α-Mo, crack deflection and branching. Flexural strength is related to volume fraction of the α-Mo and Al content. Compression tests on the 76Mo–14Si–10B alloy between 1100 °C and 1350 °C have shown excellent strength retention, and evidence of thermally activated plastic flow.