Argon matrix infrared spectra of the complexes formed between formohydroxamic acid (HCONHOH) or its isotopic analogues (HCONDOD, (HCONHOH)-N-15) and hydrogen halides (HF, HCl) have been recorded. The experimental results indicate formation of a very strong binary complex in which the hydrogen halide molecule acts as a proton donor toward the carbonyl group of formohydroxamic acid. The H-X stretches and several perturbed HCONHOH, HCONDOD, and HCO15NHOH vibrations were identified for the two HX complexes; for the HF complex two librational HF modes were also observed. Theoretical studies of the structure and spectral characteristics of the HCONHOH...HF and HCONHOH...HCl complexes were carried out on the electron correlation level with the 6-311++G(2d,2p) basis set. The calculated vibrational frequencies for the complexes present in matrixes are in good agreement with the experimental data. The calculations demonstrate also the stability of the other three binary complexes. In the cyclic structure the HX molecule acts as a proton donor for the C=O group and a proton acceptor for the NOH group, and in the two less stable structures the HX is attached to the oxygen or nitrogen atom of the NOH group.