Narrow-band resonant IR radiation promotes trans <----> cis isomerization of HONO in a Kr matrix when the O-H stretching or N=O first overtone modes are excited. The isomerization process is selective with respect to matrix sites, and the HONO concentrations in various matrix sites are unchanged, demonstrating the stability of the host cage under the resonant IR irradiation. The isomerization process is localized at the vibrationally excited molecule and possesses a high quantum yield, possibly approaching 1 for the cis to trans reaction. The present experimental results on IR-induced isomerization of HONO agree with the known molecular dynamics simulations. Our anharmonic ab initio calculations describe well the vibrational properties of both HONO isomers, including a number of observed overtone and combination bands. Especially remarkable in our opinion is the very good numerical agreement between the calculated and experimental IR absorption cross sections found for all the fundamental modes.