We report a new class of diamine hole-transporting materials (HTMs) based upon a fluorene core. Using a fluorene core, rather than a biphenyl group, leads to enhanced thermal stability, as evidenced by glass-transition (T-g) temperatures as high as 161 C for N.N'-iminostilbenyl-4,4'-fluorene (ISF). The fluorene-based HTMs have lower ionization potentials (I-p) than their biphenyl analogs, which leads to more efficient injection of holes from the indium tin oxide (ITO) anode, and higher quantum efficiencies. Devices prepared with fluorene-based HTMs were operated under thermal stress. The failure of an organic light-emitting diode (OLED) under thermal stress has a direct correlation with the thermal stability of the HTM that is in contact with the ITO anode. OLEDs based on ISF are stable to over 140degreesC.