Potassium-argon dating of glaucony, one of the few widely occurring authigenic materials in the sedimentary record, has provided a large percentage of the dates used for the calibration of the geologic time scale. This material has been considered unsuitable for 40Ar-39Ar dating because of the large losses of neutron-induced 39Ar during or subsequent to irradiation of samples. We have measured recoiled 39Ar and 37Ar from single grains of glaucony (100-350 μm in diameter). Irradiations were done both in air and in vacuo by using a microampoule technique of encapsulation. The observed in vacuo behavior of recoiled atoms can be readily understood by a simple model of their interaction with the network of very fine laminae composing the grains. When the microampoule technique is used, the integrated 40Ar-39Ar ages and their associated precision compare very favorably with conventional K-Ar results. Because glauconies are inherently heterogeneous populations, owing to their complex evolution, it is anticipated that single-grain 40Ar-39Ar dating may lead to significant refinements in the geologic time scale, especially for the Cenozoic.