The kinetics of the HO2 + ClO reaction was studied using the flash photolysis/ultraviolet absorption technique over the temperature range 203-364 K and pressure range 50-700 Torr of NS In contrast to previous work, the temperature dependence displayed linear Arrhenius behavior over the entire temperature range with the rate constant being described by the expression k(T) = 2.84 x 10(-12) exp{(312 +/- 60)/T} cm(3) molecule(-1) s(-1) Ab initio calculations of intermediates and transition states have been carried our: on the singlet and triplet potential energy surfaces. These calculations show that the reaction proceeds mainly through the ClO-HO2 complex on the triplet surface; however, collisionally stabilized HOOOCl formed on the singlet surface will possess an appreciable lifetime due to large barriers toward decomposition to HCl and HOCl. Termolecular rate calculations using ab initio parameters lead to a strong collision rate constant of similar to 5 X 10(-32) cm(6) molecule(-2) s(-1) for HOOOCl formation. This intermediate may be important under both laboratory and atmospheric conditions.