Heating samples of clays to various temperatures is a common pretreatment for identification of clay minerals by X‐ray diffraction (XRD). The exchangeable cations in the interlayer region of smectitic clays can rehydrate when the sample is cooled after the heat treatments. Because the basal spacings (d(001)) of smectites are a function of the hydration state of the interlayer cations, they can be easily misinterpreted if the hydration state is not controlled during the XRD analysis. In this report a method for preventing the rehydration of interlayer cations during XRD analysis is described, and its effectiveness is demonstrated on selected samples of clays.