Through the use of the Active Thermochemical Tables approach, the best currently available enthalpy of formation of HO2 has been obtained as Delta H-f(298)degrees(HO2) 2.94 +/- 0.06 kcal mol(-1) (3.64 +/- 0.06 kcal mol(-1) at 0 K). The related enthalpy of formation of the positive ion, HO2+, within the stationary electron convention is Delta H-f(298)degrees(HO2+)=264.71 +/- 0.14 kcal mol(-1) (265.41 +/- 0.14 kcal mol(-1) at 0 K), while that for the negative ion, HO2- (within the same convention), is Delta H-f(298)degrees(HO2-)= -21.86 +/- 0.11 kcal mol(-1) (-21.22 +/- 0.11 kcal mol(-1) at 0 K). The related proton affinity of molecular oxygen is PA(298)(O-2)=100.98 +/- 0.14 kcal mol(-1) (99.81 +/- 0.14 kcal mol(-1) at 0 K), while the gas phase acidity of H2O2 is Delta(acid)G(298)degrees(H2O2)=369.08 +/- 0.11 kcal mol(-1), with the corresponding enthalpy of deprotonation of H2O2 of Delta H-acid(298)degrees (H2O2) 376.27 +/- 0.11 kcal mol(-1) (375.02 +/- 0.11 kcal mol(-1) at 0 K). In addition, a further improved enthalpy of formation of OH is briefly outlined, Delta H-f(298)degrees(OH) 8.93 +/- 0.03 kcal mol(-1) (8.87 +/- 0.03 kcal mol(-1) at 0 K), together with new and more accurate enthalpies of formation of NO, Delta H-f(298)degrees(NO)=21.76 +/- 0.02 kcal mol(-1) (21.64 +/- 0.02 kcal mol(-1) at 0 K) and NO2, Delta H-f(298)degrees(NO2)=8.12 +/- 0.02 kcal mol(-1) (8.79 +/- 0.02 kcal mol(-1) at 0 K), as well as H2O2 in the gas phase, Delta H-f(298) (H2O2)= -32.45 +/- 0.04 kcal mol(-1) (-31.01 +/-0.04 kcal mol(-1) at 0 K). The new thermochemistry of HO2, together with other arguments given in the present work, suggests that the previous equilibrium constant for NO + HO2 -> OH + NO2 was underestimated by a factor of similar to 2, implicating that the OH + NO2 rate was overestimated by the same factor. This point is experimentally explored in the companion paper of Srinivasan et al. ( next paper in this issue).