Two specimens of interstratified ammonium-bearing mica/smectite have occurred at Aira, Kagoshima Prefecture, Japan. The mineralogical properties of the specimens were examined by chemical analysis, X-ray diffraction, thermal analysis, infrared absorption, electron diffraction and electron microscopic observation. The ammonium contents of the specimens are 0.84 and 1.45wt%, respectively. The structure of interstratification is expressed by Reichweite g = 1 having about 17 and 31% expandable layers. The IR spectra of the specimens exhibit pronounced absorption bands of ammonium ions at 1428, 2820 and 3010cm⁻¹. The morphologies of the specimens are characterized by lath- and flake-like habits, respectively.