The PES for the HO2+NO reaction was elucidated at the G2M(CC1)/6-311+G(3d,2p) level of theory. The rate constants for the forward and reverse processes were calculated under different conditions. The results show that the formation of HO+NO2 from HO2+NO is pressure-independent below 10 atm; the decomposition of HOONO to HO+NO2 is more favorable than its isomerization to HONO2 due to the high barrier for the latter process. The reverse reaction HO+NO2→HONO2 (HOONO) appears to be strongly pressure-dependent. All the predicted results agree closely with experimental values.