The rearrangement of various deuterium-labeled derivatives of pentaborane(9) in diethyl ether was studied by 11B and 2H NMR spectroscopy. Two distinct pathways for intramolecular exchange were identified at temperatures under 65 °C. One pathway allows movement from bridging positions to basal terminal positions, and a second higher energy pathway allows migration from basal terminal positions into the apex of the molecule. Exchange into and out of the apex of pentaborane appears to occur only when a substituent is one of the migrating groups. A mechanistic model for the isomerizations that conforms to the new data is suggested.