Mixed-layer illite-smectite with high degree of ordering (rectorite-like clay) and with three types of interlayer cations – K+, Na+ and NH+4, was determined in the high-sulfide epithermal mineralization of the Western Carpathian Mountains. The tobelitic portion of the clay comprises 5–10%. Ammonium was detected both by chemical analysis and by Fourier transform infrared analysis. Some of the ammonium can be assigned to a poorly bound form, not fully fixed in the illitic interlayer. The finest size fraction separated from the samples behaves differently from the rest of the sample. It contains two layers of water molecules in the expandable interlayers as determined by X-ray diffraction, whereas coarser fractions have only one water layer in the interlayers.