Abstract Geochemical research had shown that the high concentrations of boron of clay sediments are attributable to the illites. These contents of boron are either due to later absorption from the seawater or to new formation of micas with simultaneous installation of boron. Having treated clays poor of boron with solvents containing boron, it could be shown by experiment that clays can take boron from the solvents. The absorbed quantities of boron depend on the duration of treatment, on temperature, on concentration of boron, on pH of the solvents and on the mineralogical composition of the investigated clays. Dioctaheral micas absorb more boron than the further clay minerals as Kaolinite, Montmorillonite and others. Since boron practically cannot be washed away and since there is no adjustment of chemical equilibrium even after a longer treatment, adsorption cannot exist and it is to suppose that an installation of cations of boron into the lattice of mica takes place. It is probable that boron replaces the aluminium resp, the silicon in the tetrahedral position. Through these experiments it is likely that illites mostly are concerned as relies of weathering and to a less degree as new formations.