Context <p>Nitrous acid (HONO) is often associated with many air pollution events, such as the ozone hole, acid rain, and human health. Herein, we performed the theoretical studies on the structures and enthalpy of formation for the hydrated clusters HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7). Two different isomers of HONO including <i>cis</i>-HONO and <i>trans</i>-HONO were studied. Minima structures of <i>trans</i>-HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) and <i>cis</i>-HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters containing forty-eight and twenty-one were found, respectively. The hydrogen-bonded interactions between HONO and water molecules in HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters were analyzed. Enthalpies of the formation of the most stable isomers of <i>trans</i>-HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) and <i>cis</i>-HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters are predicted theoretically. These results can provide a new understanding of the atmospheric circulation of HONO.</p> Methods <p>Geometric structures and vibrational frequencies of the HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters were investigated by using the QCISD(T)/6–311 + G(3df,2p)//M06-2X/6–311 + G(3df,2p) method. Enthalpies of formation of the global minimal isomers of the HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters were calculated at the CBS-QB3 level of theory. Atoms in molecules (AIM) theory was applied to the analysis of hydrogen-bonded interactions among the HONO∙(H₂O)_<i>n</i>(<i>n</i> = 1–7) clusters.</p>