The effects of the addition of divalent metal oxides on the structure and thermophysical properties of the mixed alkali borosilicate glass system used for high level radioactive waste immobilisation have been studied. Densities, molar volumes, linear thermal expansivities and glass transition temperatures are reported and structural information has been obtained using B-11 and Si-29 magic angle spinning (MAS) NMR. Molar volume is controlled by ion size and by the formation of network polyhedra, whilst thermal expansion reflects the network rigidity. Changes in the glass transition temperature, T-S, are consistent with the different values of the heat of formation of the oxides added. Alkaline earth oxides produce an initial increase in the fraction of 4-coordinated boron, N-4, whereas PbO and ZnO reduce N-4. Si-29 NMR of the alkaline earth oxide glasses shows the presence of significant concentrations of nonbridging oxygens, consistent with a previous study of the aqueous corrosion behaviour of these glasses. PbO and ZnO reduce and ultimately eliminate the nonbridging oxygens. Estimates of the quantities of Q(4)(Pb) have been made and are shown to change with composition in the same manner as Q(4)(Me) in glasses with trivalent additions.