Well-cuttings and core samples from four Gulf Coast areas have been examined by high-resolution scanning-transmission electron microscopy and other techniques to detail the chemical, mineralogical and textural changes during diagenesis. Wells in each of the four areas exhibit a distinct smectite-to-illite (S-I) transition, but the estimated depths for the transition vary significantly: 5600 to 8800 ft for the onset of the transition, and 7500 to 13 600 ft for the completion (70–80% illite). Calculated temperatures also vary significantly: 58° to 92°C for the onset, and 88° to 142°C for the completion. No pattern is evident for characteristic temperatures of the S-I transition. These data suggest that it is inappropriate to use the S-I transition for determining absolute diagenetic temperatures in geological settings similar to Gulf Coast sequences. TEM data imply the presence of two distinctly different kinds of domains, interpreted as smectite (with some mixed-layer component?) and illite. Initially, smectite domains dominate and contain subparallel layers of variable thickness. As the transition proceeds, abundant dislocations provide avenues for diffusion of both necessary reactants (K and Al) and products (Na, Si, Fe, Mg); illite packets grow within a shrinking matrix of smectite. Variable temperatures for the transition may be due to several factors, including chemical heterogeneity of the original smectite, variation in water/rock ratio, porosity differences, diverse chemical character of available fluids, and the particular physical environment for diagenesis. It is suggested that actual depths and temperatures for the transition are primarily a function of kinetic factors associated with a reaction for which at least one of the principal phases is metastable; the transition therefore is highly dependent on local geological factors.