Using high-resolution Fourier transform spectra, a thorough analysis of the ν14c-type, ν17a-type, and ν18a-type bands of both11B2H6and10B11BH6has been carried in the 10.3-, 6.2-, and 8.5-μm spectral regions, respectively. From this analysis a large set of precise ground state combination differences withJvalues up to 36 (31) andKavalues extending to 18 (18) was derived for11B2H6(10B11BH6). These data were fitted using a Watson-type Hamiltonian leading to accurate ground state rotational constants. Anrsvalue for the B–B distance has been determined to be 1.7645(10) Å. The determination of upper state Hamiltonian constants proved to be much more difficult since the corresponding rotational levels of each of the bands are strongly perturbed by nearby dark states. To account for these strong localized resonances, it was necessary to introduce the relevant interacting terms in the Hamiltonian matrix. As a result it was possible to calculate the upper state energy levels quite satisfactorily. From these fits, estimates of the band centers and a few of the rotational constants of the resonating dark states were obtained.