Systematic study of ionization structure of WR atmospheres from the observational point of view is announced. We concentrate on line widths of different ions basing on archival data from a wide spectral range - from ultraviolet to infrared region. For a sample of 15 WN stars we search for a subtle stratification effects as seen in He II lines of different series. We discuss our results in the framework of the recent three-dimensional classification scheme for WN stars.