The fission xenon isotope dating method has been evaluated by dating four zircon samples, one sphene sample, and one carbonatite sample. The zircon and sphene samples had been dated in other laboratories by the conventional U-Th-Pb and Rb-Sr methods. Samples were irradiated with thermal neutrons in other to introduce xenon from neutron fission of 235U. After a short cooling period (before decay of 133Xe), stepwise heating of the sample and mass spectrometric analysis were used to determine the isotopic composition in the gas released at each temperature step. Fission xenon ages were calculated from the ratio of spontaneous fission xenon from 238U, and neutron fission xenon ages from 235U. The method appears to have several advantages over the convention U-Xe technique: (1) Isotope ratio measurements are inherently more precise than measurements of absolute quantities. (2) If a geological distrubance has caused loss of fission xenon, a valid date can still be obtained using the low-mobility xenon released at high temperatures. (3) In some samples, dates of disturbance and initial formation can be measured.