The unique high-resolution feature offered by N-14 magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their N-14 quadrupole coupling parameters, i.e., C-Q, the quadrupole coupling constant, and eta(Q), the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)(2)MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different N-14 MAS NMR spectra from each of which two sets of characteristic N-14 C-Q and eta(Q) values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)(2)Mo2O7, (NH4)(6)Mo7O24 center dot 4H(2)O, and (NH4)(6)Mo8O27 center dot 4H(2)O also give rise to distinct and characteristic N-14 MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)(6)Mo7O24 center dot 4H(2)O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (nu(r) = 1500-3000 Hz), all N-14 MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different N-14 MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)(2)MoO4 in humid air. Finally, by a combination of the N-14 and Mo-95 MAS NMR experiments performed here, it has become clear that a recent report of the Mo-95 MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)(2)MoO4 are erroneous because the sample examined had decomposed to (NH4)(2)Mo2O7.