A series of tellurite glasses containing TeO2, ZnO, and Na2O and doped with 1000 ppmw of Er2O3 has been synthesized. The series displays good thermal stability. Based on thermal analysis results, we chose to investigate the viscosity in the temperature range 310-395 degreesC, using Al as the best material for plates in the rheological measurements. First, the viscosity vs. strain (strain sweep) at fixed temperatures and angular frequencies was investigated. Then, having fixed the strain value, the viscosity curve vs. angular frequency was determined. Finally, the viscosity-temperature curve of all the glasses prepared was determined and fitted by different equations, including the viscosity value at the glass transition temperature, T-g, measured at a heating rate of 2 degreesC/min. The best fit was obtained using the Cohen-Grest equation. The stability of the glasses was determined by comparing DeltaT values (DeltaT = T-x - T-g, with T-x being the onset crystallization temperature). For a glass containing 10 mol% Na2O, DeltaT reached 176 degreesC. The crystalline phases formed on heating were always TeO2, Na6ZnO4, and Zn2Te3O8.