The purpose of this investigation was to determine the character of the low-angle skewness of the illite clay mineral's first order basal diffraction peak. This skewness is generally present in varying amounts on diffraction diagrams of oriented aggregates made from illitic sediments, sedimentary rocks, and soils ... A secondary objective in this study was to determine whether the amount and character of the low-angle skewed portion of the illite (001) diffraction peak is related to the cation-exchange capacity of the clay and how the skewness influences the phases formed in the clays upon ignition to high temperatures. The types of materials represented by the samples selected for study are a marine shale, a glacio-lacustrine silt, two weathered shales, and a typical underclay. Selection was made on the basis of illite content and amount of skewness of the illite (001) peak.