In fiber optics, the efficiency of light transmission depends on the perfection of total internal reflection at the interface between the core and clad of the fibers. Any loss due to light scattering at this interface can reduce the efficiency of light transmission. In this investigation, secondary ion mass spectrometry, surface analysis by laser ionization, and transmission electron microscopy have been used to characterize the interface between the core and clad of various glass systems used in coherent fiber optic devices. Interfaces with highly efficient, total internal reflection result from ‘‘compatible’’ glass combinations. Such systems have been found to have interface diffusion zones considerably narrower than systems exhibiting poor compatibility.