Mechanical spectroscopic methods and first-principles density functional calculations were applied to attempt a quantitative analysis of both atomic structure and viscous behavior of Si3N4 grain boundaries. In particular, the effect on the intergranular structure/viscosity of small fractions of selected anion/cation dopants was examined in comparison with the undoped polycrystal. From the point of view of mechanical spectroscopy, emphasis was placed on the morphologic analysis, as a function of frequency of oscillation, of a relaxation peak that originates from grain boundary sliding. The morphologic characteristics of the grain-boundary peak clearly revealed the presence of significant chemical gradients among different grain boundaries for particular dopants (e.g., Cl and Ba). On dopant addition, a reduction In activation energy for viscous intergranular flow was observed which broadened the grain-boundary peak. Chemical inhomogeneities also broadened the peak shape by generating a spectrum of activation energies. First-principles density functional calculations were conducted for cluster fragment models representative of the amorphous SiO2 intergranular film. The results explicitly showed the mechanism by which the respective dopants break bonds in the host, an action that directly reduces the viscosity of the SiO2 film. These complementary theoretical studies assist understanding and atomic-scale rationalization of the differences in segregation behavior of different dopants incorporated into the SiO2 film.