A kinematic viscosity of Co-B master alloys, containing 2, 4, 6 and 8% B, is investigated depending on time of isothermal holding. An instability of the viscosity values is revealed that is independent of the isothermal holding time after passage through the liquidus line. The critical temperatures of liquid alloy pass to equilibrium state are ascertained. The equilibrium structure obtained offers the stability of properties of the liquid system and solid one after crystallization.