The 0.5Na₂S + 0.5[xGeS₂ + (1 – x)PS_5/2] mixed glass former (MGF) glass system exhibits a nonlinear and nonadditive negative change in the Na⁺ ion conductivity as one glass former, PS_5/2, is exchanged for the other, GeS₂. This behavior, known as the mixed glass former effect (MGFE), is also manifest in a negative deviation from the linear interpolation of the glass transition temperatures (T_g) of the binary end-member glasses, x = 0 and x = 1. Interestingly, the composition dependence of the densities of these ternary MGF glasses reveals a slightly positive MGFE deviation from a linear interpolation of the densities of the binary end-member glasses, x = 0 and x = 1. From our previous studies of the structures of these glasses using IR, Raman, and NMR spectroscopies, we find that a disproportionation reaction occurs between PS_7/2^4- and GeS₃^2- units into PS₄^3- and GeS_5/2^1- units. This disproportionation combined with the formation of Ge₄S₁₀^4- anions from GeS_5/2^1- groups leads to the negative MGFE in T_g. A best-fit model of the T_gs of these glasses was developed to quantify the amount of GeS_5/2^1- units that form Ge₄S₁₀^4- molecular anions in the ternary glasses (∼5–10%). This refined structural model was used to develop a short-range structural model of the molar volumes, which shows that the slight densification of the ternary glasses is due to the improved packing efficiency of the germanium sulfide species.