MnO containing soda-lime silicate glasses were vitrified and nitrogen was introduced into them. The prepared glasses were subject to density measurement and alkaline durability tests. MnO addition caused poorer alkaline durability in 2N NaOH solution at 82-degrees-C. Needle-like precipitations were observed on the MnO containing glass surface after 96 h durability tests. Glass components were found to dissolve selectively in the alkaline solution in the order SiO2 > CaO > MnO. The density of the obtained glasses increased linearly with increasing amount of nitrogen content in the glasses. A considerable improvement in alkaline durability of the glasses by nitrogen addition was observed, and its effect was larger for the MnO-free glasses.