Intramol. interactions between the unoccupied p-orbital of the B atom and the s- (Me) or p-orbital (Ph) of the 7-endo-substituent lead to an unusual stability of the chair-chair conformation in 3-borabicyclo[3.3.1]nonanes. X-ray anal. of 3,7a-dimethyl-3-borabicyclo[3.3.1]nonane (7) and 3-methyl-7a-phenyl-3-borabicyclo[3.3.1]nonane (8), and single point ab initio calcn. (B3LYP/6-31G*) of 8 confirmed the existence of this specific attractive interaction.