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Pentium phosphate based glasses with the general formula (21-x) Na2O-xMgO-40P2O5-35ZnO-4CuO, x= 1, 2, 3, 4, 5, and 6 mol%, where the O/P ratio was varied of values, were prepared using a conventional melt quenching technique. The structure and valence states of the magnesium ions in these glasses were investigated using density measurements, infrared spectroscopy and XRD. Infrared spectra indicate that phosphate network is depolymerized with progressive substitution of alkali earth content and inform the characteristic vibrations of PO4 3-units. The depolymerization of metaphosphate chains are described by the decrease of Q2 tetrahedral sites allowing the formation of pyrophosphate groups (Q1) revealed by spectroscopic investigations. The density and the molar volume show opposite trends with increasing MgO.