Quantum chemical calculations at B3LYP and CCSD(T) levels have been performed to investigate the effects of X (X = H 2 O, (H 2 O) 2 , NH 3 , and H 3 NÁÁÁH 2 O) on HNO 3 formation by the direct (HO 2 + NO ! HNO 3) and indirect (N 2 O 4 + H 2 O ! HONO + HNO 3) reaction of HO 2 + NO. The results show that, for the direct H 2 O-assisted reaction, the entrance of H 2 OÁÁÁHO 2 and NO is more important than the three other channels of HO 2 ÁÁÁH 2 O + NO, H 2 OÁÁÁNO + HO 2 , and NOÁÁÁH 2 O + HO 2. In the H 2 OÁÁÁHO 2 + NO reactions, (H 2 O) 2 , NH 3 , and H 3 NÁÁÁH 2 O have also been taken into account and substituted in place of H 2 O; however, their contributions are negligible compared with the H 2 OÁÁÁHO 2 + NO reaction. It is noted that the atmospheric gas-phase reaction of H 2 OÁÁÁHO 2 + NO is not only competitive with the HO 2 + NO 2 ! HNO 3 reaction but can also compete well with the NO 2 + HO reaction during the day and night at 298 K. Unlike the direct reaction assisted by X, the catalytic effect taken from X can be neglected in the indirect reaction of N 2 O 4 + H 2 O ! HONO + HNO 3. However, theoretical results of the direct and indirect reaction of HO 2 + NO above may be the main reason why the yield of HNO 3 formation in HO 2 + NO reaction increases experimentally in the presence of water.