Viscosity as a function of temperature and nitrogen content and the glass transition temperature (T-g) were investigated in 20La-6OSi-20Mg-O-N glasses with nitrogen contents varying from 0 to 28 eq.% (e/o) using compressive creep and dilatometric experiments. Although T-g obtained from dilatometry were 6-12 degreesC lower than the lower limit of the transition temperature range from creep, both methods revealed identical and linear dependencies of T-g on N content. The average activation energy was 1184 +/- 36 kJ/mol and viscous flow remains the deformation mechanism in all glasses regardless of nitrogen content. Addition of 28 e/o N in oxide glass resulted in an increase of T-g by 94-105 degreesC and an increase in viscosity of around 5 orders of magnitude. These changes are greater than those reported in similar Al-containing glasses. Linear increase of T-g and compactness of the glass with nitrogen content result from enhanced cross-linking of the glass network. (C) 2005 Springer Science + Business Media, Inc.