The structure of gaseous 2,4-ethanotetraborane(10), B4H8(CH2)2, more commonly referred to as dimethylene-tetraborane, has been determined by electron diffraction and ab initio computations. The results confirm that the molecule consists of a tetraborane(10) cage substituted at the "wing" boron atoms, B(2) and B(4), by a bridging "ethene", C2H4, moiety. The molecular symmetry is C2v, the conformation about the C-C bond being eclipsed. Salient structural parameters (r(a)) of the experimental geometry are r[B(1)-B(2)] (hinge-wing) = 189.5(3), r[B(1)-B(3)](hinge-hinge) = 172.9(17), r(B-C) = 159.8(3), and r(C-C) = 156.8 (fixed) pm; B(1)B(2)B(3) = 54.2(5)degrees, and the dihedral ("butterfly") angle between the planes B(1)B(2)B(3) and B(1)B(4)B(3) is 100.8(2)degrees. These values agree well with the ab initio (MP2/6-31G*) optimized molecular geometry and are supported by comparison of the calculated (IGLO) H-1, B-11, and C-13 NMR chemical shifts, using both the MP2/6-31G* and GED geometries, with the experimental NMR data.