Abstract <p>The effect of alloying elements Nb and Mo on the viscosity of the 84KKhSR Co alloy melt is investigated using the oscillating crucible method. The temperature dependences of the viscosities of the melts are shown to coincide upon heating and cooling, to be monotonic, and to be described by Arrhenius-type dependences. Alloying with 3 wt % Mo or Nb is found to increase the viscosity of the base Co–Cr–Fe–Si–B alloy, with Nb alloying having a more pronounced effect. Alloying with niobium from 1 to 3 wt % is shown to be accompanied by a pronounced linear increase in the viscosity and the activation energy of viscous flow; for alloying with molybdenum, the concentration-induced change in the viscosity and the activation energy depends on its content in the alloy. For an alloy with 1 wt % Mo, the viscosity remains virtually unchanged compared to the Co–Cr–Fe–Si–B matrix alloy, and the activation energy of viscous flow even slightly decreases. A further increase in the molybdenum concentration in the alloy is also accompanied by a monotonic increase in both the viscosity and the activation energy of viscous flow. The minimum threshold values of temperature (<i>T</i> = 1080°C), drawing speed (<i>V</i> = 4 m/min), and viscosity (ν = 27 × 10^–7 m²/s) that ensure the production of amorphous wires from the 84KKhSR alloy by the Ulitovskii–Taylor method are determined. When analyzing data on the concentration and temperature dependences of viscosity, we have determined the optimum Co alloy compositions and quenching conditions: alloys with 2 wt % Nb and 3 wt % Mo have the same viscosity (27 × 10^–7 m²/s), and the drawing temperature of microwires is 1100°C. Amorphous wires 130–150 μm in diameter and more than 50 m in length with a high level of mechanical properties are fabricated. The obtained results indicate a high glass-forming ability of Co–Cr–Fe–Si–B (84KKhSR) alloys additionally alloyed with 2 wt % Nb or 3 wt % Mo.</p>