The equilibrium concentration (C-o) of spinel was measured in 16 high-level waste (HLW) glasses as a function of temperature (T). Glasses were formulated by increasing or decreasing concentrations of Al2O3, Cr2O3, Fe2O3, Li2O, MgO, Na2O, or NiO, one-at-a-time, from a baseline composition. Data were fitted using the quasi-ideal-solution relationship between C-o and T. The coefficients of this relationship were expressed as functions of glass composition using first-order approximation. All glass components had an effect on liquidus temperature (T-L), but only NiO and Fe2O3 had a significant impact on spinel concentration below T-L. The temperature at which Co had a given value was also expressed as a function of glass composition. These results can be used to optimize a HLW glass formulation to meet a constraint of either no spinel or a limited spinel fraction in the melter. In addition, the measurement of the size of spinel crystals and subsequent calculation of crystal number density (n) showed that Cr2O4 and Al2O3 increase n.