The clay fractions of 24 Neuse River Estuary bottom samples were analyzed by X-ray diffraction techniques to determine the clay mineral content and to note any diagenetic changes in the clays as they were introduced into a brackish-water environment. Kaolinite and "chlorite" were present in all 24 samples and illite in 18. Kaolinite was by far the dominant clay mineral being introduced into the estuary, but it decreased considerably relative to "chlorite" and illite as increasingly saltier water was encountered downstream. "Chlorite" increased downstream becoming the dominant mineral at the lower end of the estuary. Illite, though very subordinate to both kaolinite and "chlorite" throughout the estuary, showed a definite increase in the last few downstream samples and appeared to be increasing more rapidly than "chlorite." The "chlorite" was thermally unstable and collapsed to a 10 A structure when heated at 400 degrees C for one hour; however, it was like normal chlorite in retaining the 14 A basal spacing when treated with ammonium salts or glycerol.