The properties of the first excited state of I m have been studied with a mass-separated Te 129m source. The energy of this state, as determined from the conversion-electron spectrum, is JS 7 =27.72d=0.06 keV. This differs considerably from the often quoted value of 26.8 keV, but it agrees very well with the value recently published by Bemis and Fransson. An accurate half-life value ri/ 2 =16.8±0.2 nsec was determined from delayed /3e~ coincidences. A total conversion coefficient a=5.3±0.3 was obtained from the relative intensities of the 27.7-keV gamma rays and the 27.4-keV x rays that accompany the isomeric transition in Te m . The contributions of these rays were separated by comparing Mossbauer spectra obtained with the 70-min and 33-day activities of Te 129 . Some remarks are made about the relative merits of these activities for use in Mossbauer sources.