Diagnostic near-infrared spectral features have been identified for minerals with ammonium (NH (super +) 4 ) bound in the crystal structure. Spectral bands near 2.12, 2.02, and 1.56 mu m are characteristic of synthetic and natural NH 4 -bearing minerals. Laboratory spectra of NH 4 -bearing minerals are distinct from spectra of OH-bearing minerals and have diagnostic shifts in band position among different mineral types. NH 4 -bearing minerals were detected at several mineralized hot-spring deposits in the western U.S. by means of hand-held and airborne instruments. Near-infrared detection of NH 4 -bearing minerals may provide useful information for prospecting for certain ore deposits and may provide a better understanding of the nitrogen cycle within geologic environments.