Thermal isomerisation of steroid boranes derived from cholest-2-ene and -5-ene results in the migration of the boron atom from one ring to another with the establishment of an equilibrium. The most favoured position is the 3β(equatorial); next is the 2α-position (equatorial). The results support Brown's hypothesis of steric control and of the establishments of a thermodynamic equilibrium during isomerization.