Changes in the structure and properties of xNa 2 O-yTiO 2-(1-x-y)SiO 2 glasses have been studied with regards to composition through Raman spectroscopy, Brillouin light scattering, and dilatometry. Twelve glasses with compositions x=10, 15, 20, and 25, and y=4, 7, and 10 were prepared through a traditional melt quench method. Na 2 O was found to increase the density, refractive index, coefficient of thermal expansion (CTE), bulk modulus, and Poisson's ratio of glasses, and to decrease Young's modulus and shear modulus. Density, refractive index, Young's modulus, shear modulus, bulk modulus and Poisson's ratio increase with the increase of TiO 2 content. CTE was found to increase with the initial addition of 4 mol% TiO 2 and to remain constant with further additions, regardless of the Na 2 O content. Changes in the Raman spectra with composition have been discussed and related to the changes in resulting properties. Introduction: Titanium dioxide is a common minor additive in minerals and natural glasses, and it is used in commercial glasses for its optical and thermal expansion properties. It is of interest to both geologists and glass scientists because of the strong impact small quantities of TiO 2 can have on mechanical, optical, and thermodynamic properties [1-13]. Understanding the role TiO 2 plays in glass systems is complicated by the fact that Ti 4+ can occupy four-, five-, and six-fold coordination states, in tetrahedral, square pyramidal, and octahedral geometries, respectively. The coordination state that Ti adopts is dependent on the chemistry of the surrounding glass, and a change of coordination can result in TiO 2 having a completely different effect on the resulting properties. Alkali-TiO 2-SiO 2 glasses provide a convenient composition space to observe the change in behavior of TiO 2 from the binary TiO 2-SiO 2 [14]. Glasses of these compositions have been the subject of numerous studies designed to explore the structural environment around Ti. The coordination and local structure of TiO 2 containing glasses and minerals were extensively examined through XANES and XAFS by Farges et al. [15-19]. The structure of these glasses has also been examined through other means, including infrared spectroscopy [20-22], Raman spectroscopy [20,23-27], pulsed neutron scattering [28], and a variety of X-ray techniques [29-31]. A general agreement has emerged from these studies that many of the property changes caused by TiO 2 can be explained through an increase in the medium-range order. Farges et al. proposed a percolation model for a Na 2 O-TiO 2-2SiO 2 glass with pockets of 3-6 Å in size of high alkali concentration surrounded by a [5] Ti-rich region, which is then surrounded by areas with lower concentrations of network modifiers [16]. Henderson et al.'s later study of the same system, xTiO 2-(1-x)Na 2 SiO 3 glasses, showed that [4] Ti occurs at low Ti contents and gradually transitions to [5] Ti as the Ti content is increased to ~14 wt% [31]. Inoue