Calculations using a variety of high-level theoretical procedures predict a heat of formation (Delta H degrees(f298)) for the tert-butyl radical of 57 +/- 7 kJ mol(-1), which is slightly higher than recent experimental recommendations (51.5 +/- 1.7 and 51.8 +/- 1.3 W mol(-1)). The heats of formation for the tert-butyl cation (716 +/- 10 kT mol(-1)) and isobutene (-14 +/- 8 kJ mol(-1)) predicted by the same theoretical methods are also slightly higher than the experimental values (711.5 +/- 0.2 and -16.9 +/- 0.9 kJ mol(-1), respectively), while the proton affinity of isobutene (802 +/- 4 W mol(-1)) is in very good agreement with experiment (801.7 +/- 1.4 kJ mol(-1)). On the other hand, the calculated ionization energy of the tert-butyl radical (6.81 +/- 0.06 eV) is significantly higher than two experimental values (6.58 and 6.70 eV), suggesting that a reexamination may be desirable.