Abstract The postdepositional history of clay minerals is critical to understanding the diagenetic evolution of sedimentary rocks. Diagenetic changes occurring in sedimentary K-bearing clay minerals, known as illitization, are routinely identified by a changing trend in the mineralogy from smectite to illite. This change is typically gradual, with the smectite-rich components changing into illite-rich components affected by the addition of K, generally occurring during progressive burial. Understanding of illitization is also germane to the use of authigenic illite as temperature and age indicators of the thermal history of sedimentary rock sequences. Oxygen and hydrogen isotope geothermometry and radiogenic isotope dating (by K-Ar or other methods) of authigenic illite represent tools that can potentially help decipher the thermal histories of sedimentary basins. For these analytical techniques to be meaningful, a particularly adequate mineral separation and an accurate mineral identification are necessary, which may be difficult and not systematically successful because of the small sizes and varied origins of the studied minerals. Combined with detailed X-ray analyses and electron microscope observations, stable and radiogenic isotopic data have proven useful in distinguishing burial-induced from hydrothermally-induced temperature increases in sedimentary sequences. Some of the difficulties in the application of isotopic methods to separate claysized materials (which may also contain other silicates of identical size) can arise from uncertain values of the isotope fractionation factors of oxygen and hydrogen and from mixtures of detrital and authigenic components in the analyzed size fractions. Conflicting isotopic data from illite-rich size fractions may also result from difficulties associated with demonstrating chemical and isotopic equilibrium between minerals and fluids, especially in the low-temperature domain. The intimate physical association of detrital and authigenic illitic particles of varied sizes, which may not be physically separated, can also contribute to the drawbacks. In order to provide a coherent overview of the input of isotope studies to the understanding of illitization—and therefore of the thermal history of sedimentary basins—burial-induced and more sporadic rock-hot fluid interactions, such as those of Northern Germany, Paris, East Slovak, and of the Mahakam Delta, are evaluated on the basis of combined mineralogical and isotopic databases of the illitic and associated materials.