The hydroxyl stretching region of two montmorillonites (SWy-1, Crook County, Wyoming, USA, and JP, Jelšový Potok, Slovakia) and a ferruginous smectite (SWa-1, Grant County, Washington, USA) have been analysed using decomposition and curve-fitting. The positions and integrated intensities of component bands were correlated with the nearest cationic environment of the OH groups, and the populations of the octahedral sheet were calculated. The differences between the octahedral coefficients obtained and those from chemical analyses were 0.07 or less. This method provides reasonable contents of octahedral cations for the smectites with various iron contents, containing no other admixtures absorbing in the 3700-3500 cm–1 region. The cation distribution was found to be not completely disordered. Though some preferential pairing has been observed, no general rule could be established.