Total fusion40Ar/39Ar ages have been determined on 22 samples, including biotite, muscovite, sanidine, hornblende, plagioclase, pyroxene, and whole rock samples of volcanics. The samples have conventional K‐Ar ages ranging from 14 m.y. to 2000 m.y. The neutron irradiations for the40Ar/39Ar dating were made in the HIFAR reactor, using the standard biotite GA1550 as the flux monitor. The40Ar/39Ar total fusion ages of 14 samples agree to within 2 percent with the conventional isotope dilution K‐Ar ages. The remaining 8 give measured40Ar/39Ar ages from 3 to 9.5 percent greater than the conventional K‐Ar ages. These systematic discrepancies are due to significant neutron flux gradients in the reactor which become apparent because larger amounts of sample, compared with the amount of flux monitor, were used in these cases. With samples laid out in the reactor can as in the present study the precision of the40Ar/39Ar ages is about 2 percent (standard deviation) in the ideal case. Correction factors for interfering argon istopes in HIFAR have been determined: (39Ar/37Ar)ca= 7.44 (± 0.24) x 10‐4; (36Ar/37Ar)ca= 3.05 (± 0.10) x 10‐4; (40Ar/39Ar)K= 0.28±0.03.