Large refractive index variations have been recorded by combining micro-infrared reflectance spectroscopy with simulations of interference fringes measured at the anode side of thermally poled sodium-rich boro-phosphate-niobate glasses. The method developed in this work is effective for probing the spatial distribution and depth-profile of refractive index in the non-linear optical layer. It was found that the refractive index decreases and the molar volume increases with thermal poling, leading to a net increase in molecular electronic polarizability in the non-linear optical layer of the poled glass. (C) 2009 Elsevier B.V. All rights reserved.