The molecular geometry of boratrane has been determined by gas-phase electron diffraction and quantum chemical computations at the Becke3-Lee-Yang-Parr (B3LYP)/6-311+G(d) levels of theory. The B3LYP/6-311+G(d) force field was used to generate the initial set of vibrational amplitudes. The electron diffraction study gave the following bond lengths (r g , Å), bond and dihedral angles ( α, deg): r(B-N)=1.846(42), r(C-N)=1.466(18), r(B-O)=1.415(27), r(C-C)=1.507(27), r(C-O)=1.422 (ass.), NBO=98.7(21), BOC=106.8(27), BNC=102.0(14), (B-O)=33.9(40), (B-N)=-11.5(26). This electron-diffraction study showed that 5-membered rings have a distorted C-envelope conformation.