The structure, stability and vibrational spectrum of the binary complex between HONO 2 and H 2 O have been investigated using ab initio calculations at SCF and MP2 levels with different basis sets and B3LYP/6-31G(d,p) calculations. Full geometry optimization was made for the complex studied. It was established that the hydrogen-bonded H 2 O...HONO 2 complex has a planar structure. The corrected values of the dissociation energy at the SCF and MP2 levels and B3LYP calculations are indicative of relatively strong OH...O hydrogen-bonded interaction. The changes in the vibrational characteristics (vibrational frequencies and infrared intensities) arising from the hydrogen bonding between HONO 2 and H 2 O have been estimated by using the ab initio calculations at SCF and MP2 levels and B3LYP/6-31G(d,p) calculations. It was established that the most sensitive to the complexation is the stretching O H vibration from HONO 2 . In agreement with the experiment, its vibrational frequency in the complex is shifted to lower wavenumbers. The predicted frequency shift with the B3LYP/6-31G(d,p) calculations (-439 cm - 1 ) is in the best agreement with the experimentally measured (-498 cm - 1 ). The intensity of this vibration increases dramatically upon hydrogen bonding. The ab initio calculations at the SCF level predict an increase up to five times; at the MP2 level up to 10 times and the B3LYP/6-31G(d,p) predicted increase is up to 17 times. The good agreement between the predicted values of the frequency shifts and those experimentally observed show that the structure of the hydrogen-bonded complex H 2 O...HONO 2 is reliably.