Single crystal plates of cassiterites cut parallel to z-axis were used for IR messungen. Generally two types of IR absorption spectra were found. Type I is represented by synthetic cassiterites, type II by natural cassiterites. The individual absorption bands of natural cassiterites, especially in the 3320-3370 cm-1 region are probably correlated with OH groups that are coordinated to different trace element (Ti, Fe,, Nb, Ta, W) or cation vacancies.