The structure of gaseous 1-(difluoroboryl)pentaborane(9), 1-(F2B)B5H8, has been determined by electron diffraction. The results confirm that the molecule consists of a pentaborane(9) cage substituted at the apical boron atom, B(1), by a difluoroboryl group; the BF2 moiety is free to rotate about the exo B-B bond. Salient structural parameters (r(a)) are r(B-B)(base-base) = 181.2(6), r(B-B)(base-apex) = 170.6(4), r(B-B)(apex-exo) = 167.6(7), r(B-F) = 132.2(3), r(B-H(t)) = 1 19.5(13), and r(B-H(b)) = 138.9(11) pm; FBF = 115.4(6)degrees, B-H(t) "rise" (above basal plane) = 1.9(33)degrees, and B-H(b) "dip" (below basal plane) = 67.7(29)degrees. These conclusions are supported by ab initio (MP2/6-31 G* or DZP level) optimizations of the molecular geometry and by comparison of the calculated B-11 NMR chemical shifts [IGLO(DZ)//GED level] with the experimental NMR data.