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Soda-lime silicate glass is the fundamental base glass for many technologically important oxide glasses, and it has been used as window glass since the Roman Empire. Mixed-cation silicates also are useful models of the structure and dynamics of basaltic magmas and mantle melts. The diffusivity of Na + in silicate melts and its variation with composition play key roles in melting behavior. This property also depends strongly on the composition and framework structures of glasses and melts and on the presence of other types of nearby cations that often impede Na motion; this is known as a mixed cation effect. Structure-dependent silica activity also controls the composition of melts in equilibrium with mantle peridotite. Despite its importance, little is known about the detailed atomic structure and the degree of cation mixing in Ca-Na silicate glasses and melts with varying composition. Most modeling efforts assume a random distribution of these cations. Here, we use 17 O magic angle spinning (MAS) and triple quantum magic angle spinning (3QMAS) NMR to show nonrandom distributions of the network-modifying cations Na and Ca in Ca-Na silicate glasses, by probing the atomic configurations around the nonbridging oxygens. Nonrandomness in Na-Ca mixing, in particular the prevalence of Na-Ca pairs in this system, was clearly observed in 17 O MAS and 3QMAS NMR where several nonbridging oxygen peaks such as Na-O- [4] Si, and mixed peaks ({Na,Ca}-O- [4] Si) are partially resolved. The observed fractions of Na-O- [4] Si are smaller than those predicted by random distributions of Na and Ca, suggesting preference to dissimilar pairs. There are also considerable interactions between bridging oxygens and charge-modifying cations, which supports rather homogeneous distribution of such cations. 23 Na MAS NMR spectra at high field (14.1 T) provide information on chemical shift distributions with relatively small perturbations from quadrupolar broadening and provide further support for significant Na-Ca mixing. The results given here partly account for reduced Na diffusion when mixed with Ca, and together with nonrandom distributions of nonbridging oxygens, lead to negative deviation in silica activity. This also contributes to atomistic models of several magmatic processes.