A theoretical model for relating the interface viscosity to the creep behavior of highly refractory ceramics is proposed. Experimental confirmation of the model is given by monitoring the effect of systematic modifications in the interface viscosity on the creep behavior of Si₃N₄/SiC composites. A model material was selected for this basic study with a continuous film, consisting of glassy SiO₂, at the interface, whose bulk anion composition was varied by incorporating increasing amounts of glass-network modifier ions (i.e., F or Cl anions). Internal friction measurements allowed to experimentally evaluate the micromechanical response of the internal interfaces, when subjected to an externally applied shear stress. Starting from this important experimental information, the short-term macroscopic creep behavior of the polycrystal could be predicted and consistently related to microscopic interface viscosity parameter.