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Smectite clays are implicated in solute trapping in natural and engineered processes. Here we evaluated 23Na solid-state nuclear magnetic resonance (NMR) and thermogravimetric analysis-coupled mass spectrometry (TGA–MS) for quantitative analysis of a hydrated natural Na-montmorillonite equilibrated at different relative humidity (RH). Using X-ray diffraction, we determined predominantly large-sized (∼1.55 nm) interlayers at 93% and 75% RH, 2:1 ratio of medium-sized (∼1.23 nm) to large-sized interlayers at 55% RH, and 2:1 ratio of small-sized (<0.96 nm) to medium-sized interlayers at 11% RH. Informed by simulated NMR of differently hydrated model Na-MONT systems, the experimental 23Na NMR data revealed only fully hydrated Na+ populations at 93% RH, a 2:1 ratio of partially hydrated (outer-sphere) to mineral-bound (inner-sphere) Na+ populations at 55% RH and, remarkably, a near-equal proportion of these latter two Na+ populations at 11% RH. Between 93% and 11% RH, the TGA–MS data captured a 57% increase in tightly bound waters (water loss at 100–300 °C) but only a 22% decrease in freely exchangeable waters (water loss below 40 °C). The addition of exogenous NaCl altered the aforementioned hydration behaviors, particularly at low RH. Our findings of persisting hydrated environments despite interlayer collapse implied water populations incongruent with predictions from smectite interlayer nanopore size distributions.