The mid-range structure of glasses of the glass-forming system xNb2O5–(60 – x)P2O5–40Na2O, with x varying from 0 to 40, has been investigated by Raman spectroscopy and electro-optical Kerr measurements. It was found that the mid-range inhomogeneities in the glasses under study vary with x and their compositions and structures dramatically differ from the gross glass composition. It is shown that inhomogeneities with the 8NaNbO3 + (Nb2O5·P2O5) composition are responsible for the electro-optical sensitivity of the glasses. The electro-optical structural elements of the inhomogeneities of this type are assumed to be fragments of the quasicubic lattice of crystalline NaNbO3 demonstrating a rather high electro-optic Kerr coefficient. The obtained results are discussed in terms of the concepts of constant stoichiometric groupings and crystal motifs.