We use infrared-visible double resonance overtone excitation to promote HOCl molecules to single, well-characterized rotational levels of high OH stretching states just above the HOCl-->HO+Cl dissociation threshold on the ground potential energy surface. Double resonance spectra are monitored by laser induced fluorescence detection of the OH dissociation products. We present here the results obtained in the 6 nu(1) region of (HOCl)-Cl-35 where we have studied states with J ranging from 4 to 25, K-a from 0 to 5 and energy up to 300 cm(-1) above the dissociation threshold. In the spectra for K-a=0-3 states, the zeroth-order (n(OH),n(theta),n(OCl))=(6,0,0) level is split by mixing with a nearby dark state. Because the two states have very different A rotational constants, their separation increases with K-a, but the effects of the mixing remain observable in the spectrum up to K-a=3. Comparison with preliminary results from (HOCl)-Cl-37, together with analysis of the rotational constants, allows us to identify the perturbing state as (4,4,2). The lack of further strong perturbations compared to the average density of states allows us to infer that most of the matrix elements for couplings between the (6,0,0) bright state and other dark states are less than similar to 0.1 cm(-1). The average intramolecular vibrational energy redistribution (IVR) rate implied by these matrix elements (2.5x10(9) s) is two orders of magnitude longer than the predictions of statistical rate theory, indicating that IVR is likely to be the rate limiting step in the unimolecular dissociation process from (6,0,0). The present work provides the spectroscopic foundation for direct time-resolved studies of the unimolecular dissociation dynamics presented in a forthcoming paper. (C) 1999 American Institute of Physics. [S0021-9606(99)00725-4].