We wish to better understand the evolution of the tellurium environment in glasses with the addition of modifier oxides. The aim is to relate these structural changes to the non-linear optical (NLO) properties of the glass. To achieve this we propose to study a series of glasses, x Al2O3 ¿ (1-x) TeO2 using neutron diffraction and 27Al NMR to determine the variation in aluminium coordination number with composition and the effect of this on the tellurium environment.