A problem of preparation of the equilibrium melt upon Co-B-Si alloy melting is considered. Kinematic viscosity as one of the most informative structure-sensitive properties of metallic melts is studied for Co-based industrial alloys, subjected to amorphization. The temperature dependences of viscosity are determined upon heating and following cooling as well as under isothermal holding for obtaining the dependences of viscosity on time. Relationship between melt preparation conditions and metal product quality is revealed. The Co-based alloys are used as the magnetically soft materials with the unique magnetic properties.