Numerous 40 Ar/ 39 Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and UintaBasins of Wyoming, Colorado, and Utah constrain ~8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40 Ar loss, and 39 Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a signifi cantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven 40 Ar/ 39 Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries.Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfi lled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced fi lled to overfi lled Tipton and Laney Members. Sediment accumulation patterns thus refl ect basin-center-focused accumulation rates when the basin was underfi lled, and supply-limited accumulation when the basin was balanced fi lled to overfi lled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ~150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fl uctuating profundal facies, which likely refl ects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fi elds entered the Green River Formation lakes from the north.Our new ages combined with existing paleomagnetic and biostratigraphic control permit the fi rst detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfi lled) lakes were initially dominant (53.5-52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fi ll lakes fi rst occurred in all Green River Formation basins ...