Glasses in the system phosphorus-selenium-tellurium have been prepared over a wide compositional range and characterized by differential scanning calorimetry and P-31 solid-state NMR techniques. The NMR data reveal clearly that Se and Te are structurally inequivalent in these glasses. P-31 spin echo NMR studies reveal that in glasses’ with fixed P/Se ratio the presence of additional tellurium does not influence the distribution of the P atoms between P-bonded and non-P-bonded structural units appreciably. Magic angle spinning NMR spectra show no evidence for the formation of any P-Te bonds but distinguish clearly between three- and four-coordinate P atoms. The results indicate that Te plays two distinct roles in these glasses:; at lower levels of incorporation, it strongly stabilizes tetrahedral Se=PSe3/2 groups, thereby resulting in significantly increased concentrations of these units. At higher Te contents, partial segregation occurs, presumably due to solubility limitations. In spite of this complexity, the entire set of NMR data can be described phenomenologically in terms of an association equilibrium Se=PSe3/2 + Te <-> Te...Se=PSe3/2, with an average equilibrium constant K-2 of 25 (mole fraction)(-1). Spin echo MMR studies reveal that the dipolar P-31-P-31 interactions are significantly weaker for the four-coordinated P atoms than for the three-coordinated P atoms. The selenium-tellurium interactions proposed for these glasses would result in a net increase in the average coordination number, accounting for significant compositional effects on the glass transition temperatures in this system. The NMR data offer a dear-cut structural rationale for these observations.