The illitization of a K-smectite in 0.5 molal KCl solution at 300°C and vapour saturation was studied to determine the nature of layers in interstratified run products formed in solutions supposed to favour the formation of illite layers. Solid products were analysed by X-ray diffraction (XRD), electron microprobe and infrared spectroscopy (FTIR), and solutions by ICP-AES and ICP-MS. The percentage of illite layers in mixed-layer clays was determined by XRD and IR spectroscopy (on NH4+ saturated experimental products). The main results are: (i) discrepancies between the Si contents measured in mixed-layer clays by electron microprobe and the percentages of supposed illite layers estimated by different techniques confirm the existence of non (glycol) expandable dehydrated K-smectite layers; (ii) infrared spectroscopy indicates that some of these layers have not fixed interlayer K+ ions and are still exchangeable (with NH4+) whereas others cannot exchange their interlayer cation; this behaviour is probably related to the layer charge deficiency and its structural location in the smectite layers; (iii) the amounts of dissolved smectite layers and crystallized non-expandable layers (illite, dehydrated smectite) in newly formed ordered interstratified clays increase with time. The high concentration of K+ and the absence of Na+ in the experimental solution does not clearly favour the formation of illite in short-time experiments (less than 6 months).