Abstract: Mixed-layer illite/smectite (I/S) clays in lower Eocene sediments of the Texas Gulf Coast decrease in expandability with depth as found in previous studies of clay diagenesis, but at the same depth the 1/S clays from sand laminae tend to be more expandable than clays from adjacent shales. The I/S clays with greater expandability from sands collapse to phases with very low expandabilities after saturation with K. This non-ideal smectite behavior indicates that many of the expandable layers in I/S clays from sand laminae possess an illite-level negative layer charge on the structure. The Greene-Kelley Li-saturation test of the I/S clays also reveals that a considerable portion of the increased layer charge deficiency is created by tetrahedral substitution. The illitization of individual smectite layers in an I/S clay is envisioned to be a two step process instead of the single step rapid transformation implied in many earlier studies. The first rate-limiting step is the creation of sufficiently large negative layer charge in the lattice, primarily by AI for Si substitution in the tetrahedral sheet. The second rate-limiting step is the supply of K to the high-charged expandable layers. The abundance of high-charged expandable layers in I/S clays from sand laminae suggests that the K supply is influenced more by competing ions in the interstitial waters than by the absolute activity of K.