Supplementing gas-phase electron-diffraction data with restraints derived from a graded series of ab initio calculations makes possible refinement of all geometrical parameters and amplitudes of vibration. By avoiding the need to fix some parameters, this technique yields structures which are more completely refined and thus have more reliable standard deviations than procedures used previously. It has been applied to the gas-phase structure of the arachno boron hydride tetraborane(10), B4H10. Salient structural parameters (rα0 structure) were found to be: r[B(1)–B(2)] 186.6(2), r[B(1)–B(3)] 173.7(5), r[B(1)–H(1,2)] 123.0(15), r[B(2)–H(1,2)] 141.7(8), r[B(1)–H(1)] 119.8(8), r[B(2)–H(2)]endo] 121.0(8) and r[B(2)–H(2)exo] 120.5(8) pm; butterfly angle 117.2(4)°. The crystal structure was also redetermined at 100 K. All gas-phase, crystallographic and ab initio structural parameters were found to be in good agreement.