The viscosity of As₂S₃-P₂S₅ melts was measured in a wide range of temperatures and concentrations. As the content of P₂S₅ increases, the viscosity of the melts decreases nonmonotonically, especially at low temperatures, in the supercooled state. This behavior of viscosity, along with the extrema in the composition dependences of the activation parameters for viscous flow, suggest that As₂P₂S₈ is chemically stable in the supercooled liquid and vitreous states.