Structures, energetics, and vibrational properties for the bimolecular complex of HONO have been studied by ab initio methods. Ten stable complexes of (HONO)(2) were found. Four of the complexes are formed between two trans isomers, three between two cis isomers, and three complexes are found between one trans and one cis isomer. All complexes involve a hydrogen bond from the OH tail of one of the subunits. The calculated interactions energies at the MP2/6-311++G(2d,2p) level vary between -10 and -20 kJ mol(-1) representing a relatively strong interaction. The structural properties of the subunits were found strongly perturbed upon complexation, and the vibrational spectra of the various (HONO)(2) complexes are predicted to show large vibrational shifts compared with the monomer spectra.