A recent investigation of the binary borosilicate glass system has led to the realization of a useful cladding material for pure fused silica core fiber optical waveguides. The feature which makes the borosilicate glass useful is that its index of refraction is sufficiently less than that of pure fused silica to allow light guidance in silica core‐borosilicate clad fibers. The previous work offered no explanation for the observed but unexpected behavior of the borosilicate refractive index. Continued studies of this glass system have now led to a quantative explanation of the refractive index behavior. Two theoretical approaches are discussed. One is based on the Sellmeier dispersion model and the other on a molar refractivity analysis. The main quantities of interest turn out to be density, composition, structure, and ionicity. An important prediction based on this work is that properly quenched borosilicate glass can have a refractive index even lower than previously observed. This prediction was subsequently confirmed and has the the practical consequence of allowing the waveguide designer greater latitude in his choice of variables. A further observation is that borosilicate glasses exhibit larger ``energy gaps'' than composite glasses such as soda‐lime‐silicates, and may therefore be more desirable from the point of view of absorption loss.