The compositional variations of the diffusivity of network-forming ions (Si, Al, O) and viscosity in silicate liquids have been modeled using percolation theory. The relative fraction of bridging oxygen (BO) and non-bridging oxygen (NBO) in a silicate liquid serves the purpose of the simplest critical variable with respect to which diffusion and viscous flow show interesting cross-over behaviors. The diffusion of network-forming ions is found to be strongly coupled to one another and show a percolation transition at 16 vol% of NBO's. The viscosities of silicate liquids also show a rigidity percolation behavior at a similar volume fraction of NBO's. These observations indicate that microscopic diffusion events are probable precursors of viscous flow. Effective medium theories have also been used to model the compositional dependence of the viscosity of silicate liquids.¿ 1997 American Geophysical Union