The potential energy surface of the HNO + NO reaction has been investigated by ab initio molecular orbital calculations at the QCISD(T)/6-311G(d,p)//UMP2/6-311G(d,p) + ZPE[UMP2/6-311G(d,p)] and Gaussian-2 (G2) levels of theory. The initial reaction step is NO association with the N atom of the HNO molecule to form the HN(O)NO intermediate, 2, overcoming the barrier 1′ of 9.5 kcal/mol. The reaction proceeds further by 1,3-hydrogen migration in HN(O)NO from nitrogen to oxygen via the transition state 3, which is much more favorable than 1,2-shift. This step is shown to be rate-determining, having a barrier of 21.6 kcal/mol. After the H shift, trans,cis-HONNO (2A″) intermediate, 5a, is formed, which rearranges to trans,trans-HONNO (2A′), 7b. Finally, the latter dissociates to give the reaction products N2O + OH. The energies of the transition states for internal rearrangements of HONNO as well as the transition state for HONNO (2A′) dissociation are calculated to be significantly lower than the rate-determining barrier for 1,3-hydrogen migration in HN(O)NO.