In order to assess the influence of tetrahedral sheet charge on the hydroxyl-stretching vibration in clay minerals, a series of synthetic Na-saponites, with increasing layer charge, was studied by infrared (IR) spectroscopy. When recorded after KBr dilution either in transmission or diffuse reflectance, the IR spectra exhibit a component at approximately 3710 cm (super -1) , the intensity of which increases linearly with increasing layer charge. When experiments are carried out without any dilution of the samples, this component is not observed, suggesting a Na (super +) /K (super +) exchange upon mixing of the sample with KBr. The spectra obtained for K-saponite confirm this exchange. This shows that the dilution procedure can lead to serious misinterpretations of IR spectra in the OH-stretching region.