Twelve samples of smectites and nine of illites, from the main Slovak deposits of various geological origin were examined by FTIR spectroscopy. Some of the <2 µm fractions were pure, but in others admixtures such as kaolinite, cristobalite, quartz, amorphous SiO₂, carbonates or boehmite were identified. The FTIR spectra confirmed that Al-rich montmorillonites were formed from rhyolites and rhyolite tuffs, while Fe-rich montmorillonites or nontronites are products of weathering of andesites and their tuffs or of basalts. The absorption bands near 828 and 754 cm^-1 revealed illite layers in rectorite and other minerals containing illite. A decrease in the intensity of the band near 620 cm^-1 and an increase of that near 420 cm^-1 is characteristic for the increasing amount of non-swelling layers in mixed-layer illite/smectite.