The dissociation dynamics of energy selected neopentane, t-butyl iodide, and t-butyl hydroperoxide ions have been investigated by threshold photoelectron-photoion coincidence (TPEPICO) spectrometry. Although the methyl loss reaction from neopentane ions producing the t-butyl ion is in competition with a lower energy methane loss channel, modeling these two channels with the statistical theory of unimolecular decay provides a 0 K dissociation onset for methyl loss of 10.564 +/-0.025 eV. This leads to a 298 K t-butyl ion heat of formation of 714.3 +/- 2.5 kJ.mol(-1), which is some 3 kJ.mol(-1) higher than the previously accepted value. The Delta H-f(298K)degrees(t-C4H9+)combined with the measured 0 K onsets for t-C4H9+ formation from t-butyl iodide (9.170 +/- 0.007 eV) and from t-butyl hydroperoxide (9.904 +/- 0.012 eV), yields 298 K t-butyl iodide and t-butyl hydroperoxide heats of formation of -68.5 +/- 2.6 kJ.mol(-1) and -233.2 +/- 2.8 kJ.mol(-1), respectively. Finally, the new t-C4H9+ heat of formation leads to a predicted adiabatic ionization energy for the t-butyl radical of 6.86 +/- 0.20 eV, and a 298 K proton affinity for isobutene of 798.8 +/- 2.5 kJ.mol(-1). The predicted ionization energy exceeds all measured values by 0.10 eV.