The IR spectra of NH4+-saturated smectites were examined in terms of their chargecharacteristics. The 4 NH4+ band near 1440 cm-1, observed in the DRIFTS spectra (obtained without use of a KBr matrix), was assigned to the vibrations of NH4+ ions compensating the negative charge of the clays. When KBr was used as a diluting matrix, the 4 NH4+ band was located at 1400 and/or 1440 cm-1. The band at 1400 cm-1, related to NH4Br, originated from the replacement of NH4+ in the clay by K+ from the KBr. For swelling clay minerals this band indicates that layers have permanent low charge density and/or variable charge. For non-swelling clay minerals, the 1400 cm-1 band characterizes the presence of variable charges only. The 4 NH4+ band at 1440 cm-1 suggests that NH4+ in the clay was not replaced by K+ from KBr and remains in the interlayer space of the clay minerals. This absorption is due to NH4+ compensating only permanent charge in the interlayers, or part of the interlayers with a high charge density. The presence of both bands at 1400 cm-1 and 1440 cm-1 in the IR spectrum suggests that the clays studied have a heterogeneous interlayer charge.