Dissociations of the O-N bond in trans-HONO and 0 0 bond in HOOH are studied using the quasiclassical trajectory approach. Despite their similarities, the results are quite different. The method seems to work well for HONO, giving results in accord with experiment. On the other hand, the classical trajectory approximation breaks down for HOOH near the reaction threshold due to the aphysical zero-point energy flow, although it is better for the higher energy states. These results show that the intramolecular couplings are quite different for the two seemingly similar systems. (C) 2003 Elsevier B.V. All rights reserved.