The complexes formed by trans- and cis-HONO isomers with nitrogen and carbon monoxide have been observed and characterized in argon matrices. Six perturbed trans-HONO vibrations and four perturbed cis-HONO vibrations were identified for both the N-2 and CO complexes. The perturbation of the OH group vibrations proves that all four complexes are hydrogen banded; the blue shifts of the CO vibrations in trans- and cis-HONO complexes as compared to CO monomer indicate the OC ... HONO structures in which carbon atoms are the acceptor sites. The strength of interaction, as evidenced by the perturbation of the OH vibrational modes of the nitrous acid, increases from cis- to trans-isomer and from nitrogen to carbon monoxide. Theoretical studies of the structure and spectral characteristics of the complexes formed between the two isomers of nitrous acid and nitrogen or carbon monoxide were carried out on the electron correlation level with the 6-31G(d,p) basis set. The binding energy and the calculated spectral parameters are in very good agreement with experimental data.