FTIR temperature dependence studies of the spectra of trans-HONO monomer trapped in an argon matrix are presented. The spectral studies show that doublets observed in the N2O, N-O stretching, NOH bending and OH torsion regions and a triplet in the OH stretch region show reversible temperature dependence; one component of each doublet and two components of the triplet exhibit larger broadening with temperature than the other component in the same region. Molecular dynamics simulations of trans-HONO isomer in an argon matrix demonstrate the existence of two types of trapping cages. The molecule trapped in a one-atom substitutional cage has more freedom of reorientational motion than the molecule trapped in a two-atom cage. The bands showing a stronger temperature dependence are assigned to the molecule in a one-atom cage and the others to the molecule in a two-atom cage.