Three smectites with various Fe content were chosen to study the proportion of the octahedral atoms in the region of stretching and bending OH vibrations. Complex band in OH stretching region was resolved into the components corresponding to OH groups coordinated to specific groupings of octahedral atoms. Proportionality of the integrated intensities of the OH components to the number of absorbing centres of given type was used to calculate the contents of cations in the octahedral sheet. The coefficients of the octahedral atoms thus estimated were found to be close to the values calculated from chemical analysis for sample with minor amount of non-smectitic phase.