The illite/smectite (I/S) identification diagram of Środon (1980), which is based on the positions of 003 and 005 reflections, was constructed assuming three even distributions of crystal thickness: 1–8 and 1–14 layers per crystal for random, and 7–14 for ordered interstratifications. At the time, these distributions were regarded reasonable for diagenetic I/S. Recent studies based on TEM and Bertaut- Warren-Averbach analyses demonstrate, however, that authigenic illites and I/S are characterized by a unique type of lognormal distribution of fundamental particles and crystal thicknesses. The present work shows that XRD data support the correctness of the lognormal crystal size distribution model in the case of illites. The 003 vs 005 diagram then was refined using the lognormal model and a new estimate of the mean crystal thicknesses for a given expandability presented by Drits et al.(in press). The new diagram was calibrated using the NEWMOD computer program. Expandabilities obtained from the original diagram are overestimated by up to 5%S with respect to the refined plot. The refined technique yields %S slightly lower than more sensitive high-angle technique, which is based on weak reflections, the positions of which are not crystal-size dependant. NEWMOD simulations of complete XRD patterns in the 2–50° 2θ range, based on parameters measured by the refined technique, produced very good fit to real XRD patterns. Minor differences are attributed to an error in estimating the LpG² factor for smectite used in NEWMOD calculations.