The Rb-Sr method can be used to date diagenetic events that have affected the illitic clays commonly found in sedimentary rocks. Diagenetic 1Md illite forms in equilibrium with pore-water Sr and appears to be a stable phase that is able to maintain a closed chemical system even under deep burial conditions. Meaningful analytical data can be obtained after careful size fractionation to separate coarse-grained, detrital 2M illite from finer-grained, diagenetic 1Md illite, followed by removal of exchangeable Rb and Sr with ammonium acetate. Rb-Sr data from the <0.2 µ fraction of widely separated samples of Upper Devonian black shales in Texas fall on a single isochron having an age of 302 ± 4 m.y. Diagenetic illite must have formed rapidly in response to activity in the nearby Ouachita orogenic belt. In the middle Pennsylvanian, none of the shales was buried more than a few hundred metres, and so clay diagenesis may have been initiated by updip migration of hot brines out of the Ouachita tectonic zone. Later burial to as much as 7 km did not result in further diagenetic changes affecting the Rb-Sr system. Coarse-grained illite (2–1 µ) from the west Texas Woodford Shale records an isochron age of the source region of 539 ± 18 m.y. This is an age not commonly encountered in rocks of North America, suggesting a derivation of Woodford clays from the southern continental landmass.