Determination of ammonium in mica clay minerals has been made by wet chemical analysis, X-ray diffraction and infrared absorption methods. After sample decomposition with hydrofluoric and sulfuric acids, ammonium is chemically determined by titration with standard acid or colorimetry with Nessler's reagent through extraction using a steam distillation unit. Basal spacing (d001) and relative intensity of infrared absorption band of ammonium (1430 cm-1) to that of hydroxyl (3630 cm-1) are closely related to interlayer ammonium ratio and ammonium content, respectively. Two empirical curves to determine ammonium in mica clay minerals are presented.