Single-crystal laser fusion 40Ar/39Ar analyses and several conventional bulk fusion 40K--40Ar dates have been used to determine the age of volcaniclastic strata within the Olorgesailie Formation and of associated volcanic and sedimentary units of the southern Kenya rift. In the principal exposures along the southern edge of the Legemunge Plain, the formation spans the interval from approximately 500 to 1000 ka. Deposition continued to the east along the Ol Keju Nyiro river where a tuff near the top of the formation has been dated at 215 ka. In these exposures, the formation is unconformably overlain by sediments dated at 49 ka. A possible source for the Olorgesailie tephra, the Ol Doinyo Nyokie volcanic complex, contains an ash flow dated at ~1 Ma, extending the known age range of this complex to encompass that of virtually the entire Olorgesailie Formation in the Legemunge Plain. These geologic examples illustrate the importance of the single-crystal 40Ar/39Ar dating technique whereby contaminant, altered, or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits. We have presented a computer-modeling procedure based on an inverse-isochron analysis that promotes a more objective approach to trimming 40Ar/39Ar isotope data sets of this type. ¿ American Geophysical Union 1990