Under compression, the As-S liquids are subject to transformations, including polymerization and metallization. We have measured the viscosity of the As-S liquids under high pressures. As a result, large viscosity variations by 4-5 orders of magnitude have been revealed. The viscosity values of the As-S liquids are moderate in the molecular state, very high in the covalent state, and low in the metallic state. Therefore, predicting the viscosity behavior in other melts under pressure is possible, which is of considerable importance for the physics of glass transition, geophysics, and material science.