The viscosity behavior of glasses with the composition (90-x)TeO2-10Bi(2)O(3)-xZnO with x = 15, 17.5, and 20 (TBZ glasses) and 80TeO(2)-(20-y)Na2O-yZnO system with y = 0, 5, and 10 (TNZ glasses) have been measured as a function of temperature using a beam-bending (BBV) and a parallel-plate (PPV) viscometer. The structure of the glass' network has been characterized using Raman spectroscopy and has been related to the viscosity temperature behavior and the fragility parameter (m) of the glasses. As the concentration of ZnO in the TBZ system (x) increases, the fragility parameter of the glass increases, whereas it decreases with an increase of the ZnO concentration (y) in the TNZ system. In both glasses, these variations in m have been related to the partial depolymerization of the tellurite network associated with the level of modifier content. The depolymerization of the tellurite network is believed to be the result of a reduction in the number of [TeO4] units and the formation of [TeO3] and [TeO3+1] units that occurs with a change in TeO2 content in the TBZ system and modifier content in the TNZ system. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.