Double-resonance overtone excitation prepares HOCl molecules in single rovibrational states above the unimolecular dissociation threshold in the ground electronic state, Detecting the OH dissociation fragments allows us to observe which reactant states lie above or below the dissocation threshold and determine that threshold to be 19 290.3+/-0.6 cm(-1). Dissociation rates from single, well-characterized eigenstates of the parent molecule exhibit fluctuations of more than an order of magnitude. (C) 1997 American Institute of Physics. [S0021-9606(97)04047-6].