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This paper presents the structural characterisation and degradation behavior of the glass system 20Na 2 O–(30 − X )CaO– X MgO–50P 2 O 5 ( X = 0, 5, 10, 15, 20, 25 and 30 mol%) wherein magnesium oxide is introduced as a partial calcium oxide substitute. This system is of interest from the viewpoint of investigating the subtle effects on structure and degradation that arise from the substitution of divalent ions (Mg 2 + for Ca 2 + ) in these glass systems. Studies such as differential thermal analysis (DTA), Fourier transform infrared (FTIR) spectroscopy and 31 P nuclear magnetic resonance (NMR) reveal little change in the glass structure. However, degradation and ion release measurements for glass discs immersed in deionised water at 37 °C over a 21 day period highlight an inverse exponential effect between glass degradation rate and MgO content. Thus, the investigated glass systems provide interesting insights into the effects of parameters such as ionic radius, atomic weight and bond length on the glass properties. From an applications perspective, these glasses offer a selective range of degradation rates from highly degradable to relatively stable systems and can potentially serve as vehicles for the delivery of magnesium ions in the clinical realm.