The compounds 2,4-(methylethano)tetraborane(10), (MeCHCH(2))B4H8 (1), and 2,4-(trans-dimethylethano)tetraborane(10), (MeCHCHMe)B4H8 (2), synthesized from B4H10 and MeCH=CH2 or trans-MeCH=CHMe, respectively, have been characterized and their molecular structures determined by gas-phase electron diffraction and ab initio computations at the MP2/6-31G* level. The equilibrium structures of 2- and 4-n-propyl-2,4-(methylethano)tetraborane, 2-Pr-2,4-(MeCHCH(2))B4H7 (3) and 4-Pr-2,4-(MeCHCH(2))B4H7 (4), Obtained as side products in the synthesis of 1, have also been characterized and their structures optimized using ab initio computations. 3 and 4 represent the first examples of trisubstituted derivatives of tetraborane(10). The diffraction patterns of 1 and 2 are consistent with heavy-atom, C2B4, cages that are only slightly distorted away from C-2v symmetry with twist angles of 0.5 and 0.8 degrees, respectively, for the C(5)-C(6) bonds about the pseudo-C-2 axis. Other structural parameters (r(a)) of the experimental geometries for 2,4-(MeCHCH(2))B4H8 and 2,4-(MeCHCHMe)B4H8 respectively, include r[B(1)-B(2] (hinge-wing) = 189.1(2) and 189.3(3), r[B(1)-B(3)] (hinge-hinge) 171.6(8) and 171.2(9), r(B-C) = 161.2(9) and 161.5(11), and r(C-C) (skeleton)= 156.2(9) and 156.7(11) pm; B(1)B(2)B(3) = 54.0(2) and 53.7(3)degrees, and the dihedral ("butterfly") angles between the planes B(1)B(2)B(3) and B(1)B(4)B(3) are 100.4(2) and 100.4(3)degrees. These values agree well with the ab initio (MP2/6-31G* level) optimized molecular geometries and are supported by comparison of the calculated (IGLO) B-11 MMR chemical shifts, using both the MP2/6-31G* and GED geometries, with the experimental NMR data. The theoretical structures of 3 and 4 are also supported by B-11 NMR chemical-shift calculations.