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 Δ_fH°_298K(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.