Partly rotationally resolved (2 + 1) resonance-enhanced multiphoton ionization spectra of the vibrational bands corresponding to the b[(II1/2)-I-2](c)6s;1 <-- X transitions in (IBr)-Br-i (i = 79, 81) for the natural abundance of isotopes have been recorded at room temperature. Vibrational bands consist of contributions from both isotopomers with varying degrees of overlap depending on vibrational quantum numbers. The band spectra were analyzed by simulation calculation. Rotational constants (($) over bar B-v’ and ($) over bar D-v’) and corresponding internuclear distances (r(v’)) for the Rydberg state (b[(II1/2)-I-2](c)6s;1) were obtained, as well as equilibrium position rotational parameters. Thus for (IBr)-Br-79, B-e’ = 0.061 61 +/- 0.000 05 cm(-1), alpha(e)’ = (2.2 +/- 0.1) x 10(-4) cm(-1) and r(e)’ = 2.371 +/- 0.002 Angstrom. Vibrational analysis based on the determination of band origins from the spectral simulations were performed to obtain omega(e)’ = 307.4 +/- 0.5 cm(-1), omega(r)x(e) = 1.0 +/- 0.1 cm(-1) for (IBr)-Br-79 and T-e’ = 56 362 cm(-1). Spectroscopic parameters for (IBr)-Br-81 are derived from the ones for (IBr)-Br-79 in a standard way (see text). Perturbation due to interaction between the Rydberg state and an ion-pair state is considered.