Pentaborane(9) derivatives of the type 1-XB5H8 are more stable when X = heavier halogen, whereas 2-XB5H8 compounds are more stable when X = lighter halogen. The reason may be found in the different π-type interactions with the B5 cage, and similar ideas explain some otherwise anomalous UB nmr chemical shifts. The catalytic isomerization of 1-IB5H8 to the far less stable new isomer 2-IB5H8 (mp -39°; bp 167° est) requires steady removal of this more volatile product. It is argued that halogen exchanges on the B5 skeleton are done best by gas-flow processes at minimal pressure, as when HgCl2 converts 1-IB5H8 to 1-C1B5H8 or SbF3 converts 2-IB5H8 to the new 2-FB5H8 (mp —63°; bp 71° est). However, the analogous synthesis of 1-FB5H8 gave a product too unstable to be isolated. The 11B nmr comparisons include all four of the 2-halogenated pentaboranes and 2-CH3-B5H8, and accurate vapor-phase infrared spectra are reported for the five known XB5H8 compounds not previously so recorded. © 1968, American Chemical Society. All rights reserved.