Cl36 was prepared by neutron irradiation of C.P. grade potassium chloride for 14 months in the NRX reactor. The disintegration rate of a purified sample of the irradiated material was determined by absolute beta counting with a 4π methane flow-type proportional counter. The number of Cl36 atoms in the sample was determined by gravimetric and mass spectrometric analysis. A value of (3.08 ± 0.03) × 105 years was obtained for the half-life of Cl36, assuming that the simple decay scheme is correct.