The reactants, products, and the transition state for the reaction NO2 + OH = NO + HOO have been investigated using ab initio molecular orbital theory at the MP2 level. Geometry optimizations and vibrational frequency calculations have been performed on all reactants, products, and the transition state at both the MP2/3-21G** and the MP2/6-311G** levels. Additional calculations have been carried out using the same levels of theory and basis sets for the reaction HOO + O = HO + O2. These calculations allow us to estimate the rate of these reactions and their importance in NOx depletion and formation processes.