Zn-Mg-phosphate (ZMP) glasses, including those with the nominal molar compositions (50 - x)ZnO xMgO 50P2O5 (0 ≥ x ≥ 50) and (60 - y)ZnO yMgO 40P2O5(0 ≥ y ≥ 60), were prepared and properties such as density, refractive index, coefficient of thermal expansion and glass transition temperature (Tg), were measured. The glass transition temperature increases by 100-150 [degree]C and the room temperature dissolution rates in water decrease by 1-2 orders of magnitude when MgO replaces ZnO while maintaining a constant P2O5 content. Glass structures were characterized by Raman spectroscopy and there were no significant changes in the phosphate anion distributions when MgO replaces ZnO. The significant changes in properties cannot be explained by a simple cation field strength argument since Mg2+ and Zn2+ have similar sizes; instead, the effect of 3d electrons on the nature of the bonds between Zn2+ ions and non-bridging oxygens on the phosphate tetrahedra must be considered.