X-ray diffraction analysis of natural and intercalated montmorillonites is usually limited to the apparent d-spacings estimated from the peak maxima in the raw data. This can lead to different estimated d-spacings and to the wrong interpretation of the measured data. In the case of X-ray diffraction, the interference function is modulated by instrumental and physical factors which lead to distortions of diffraction profiles and to the shift of the peak position. The present work deals with the detailed analysis of these effects, their corrections and their consequences for the interpretation of diffraction patterns. On the example of montmorillonite intercalated with Al-hydroxy polymers, it is shown that the difference between the d-spacing estimated from raw data and the d-spacing after the correction for angular dependent factors is about 1 Å.