The complexes formed between cis- and trans-HONO isomers and ammonia have been observed and characterized in argon matrices. Five perturbed HONO vibrations and one perturbed NH3 deformation vibration were identified for the H3N···HONO-trans complex, and one perturbed HONO vibration and perturbed NH3 deformation vibration were identified for the H3N···HONO-cis complex. The OH stretching vibration in the H3N···HONO-trans complex is ca. 800 cm-1 red-shifted and NOH bending vibration is ca. 190 cm-1 blue-shifted with respect to the trans-HONO monomer, indicating formation of a very strong molecular hydrogen bond. Theoretical studies of the structure and spectral characteristics of the H3N···HONO-trans and H3N··· HONO-cis complexes were carried out on the electron correlation level and G-311+G(2df,2pd) basis set. The calculated binding energy at the MP2 level is −40.13 and −36.39 kJ mol-1 for the H3N···HONO-trans and H3N···HONO-cis complexes, respectively. The calculated spectra reproduce very well the frequencies and the intensities of the measured spectra.