Sodium borosilicate base glasses modeled on French nuclear waste materials were prepared to test the dependence of crystallization product quantity and distribution on cesium‐ and molybdenum‐loading and glass cooling rate. Scanning electron microscopy shows the presence of micrometer‐sized domains of Mo‐rich crystalline precipitates. X‐ray diffraction patterns are complex but reveal the presence of sodium molybdates and CsNaMoO4·2H2O. 133Cs and 23Na magic‐angle spinning NMR spectroscopy exhibit distinct peaks for glassy and crystalline phases which can be quantified to obtain the identities of the individual compounds that are formed as well as the fractions of these nuclei in particular crystalline phases. In these model systems, 1 mol% Mo can be entirely incorporated into the glassy network whereas 2.5 and 5 mol% Mo produce significant quantities of crystalline phases, with little dependence on cooling rate. Cesium content appears to have a weak influence on crystallization behavior. Sodium molybdate and sodium‐cesium molybdate hydrate are the dominant devitrification phases in all cases.
