A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the Ar-40-Ar-39 dating technique can be successfully used to determine the diagenetic age of ancient sediments because Ar-39 losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic Ar-40 in nature, rather than by direct recoil as is generally assumed.