Overtone excitations may play an important role in the chemistry of HNO 3 and related species in the atmosphere. Theoretical simulations were performed using realistic potential surfaces of the overtone and combination-mode spectroscopy of HNO x species, and of the dynamics of these systems following excitation. Algorithm of the group and using ab initio MP2 potentials were in very good accord with the observed transitions for cis-HONO, trans-HONO, HNO 3, HNO 4, and HNO 3-H 2O. Even for high overtones, energy flow from the excited OH stretch is expected to be slow. The dynamics following overtone excitation was studied by classical trajectory simulations, using PM 3 potentials "on the fly". In HONO, HNO 3, HNO 4, the most efficient process following excitation was large-amplitude "shuttling" of the H atom between the oxygens. "On the fly" dynamics simulations might be a powerful new tool in the exploration of atmospherically-relevant chemistry. This is an abstract of a paper presented at the 227th ACS National Meeting (Anaheim, CA 3/28/2004-4/1/2004).