To our knowledge, the length of the U¿H single bond has never been accurately measured, at least not in a molecular compound by single-crystal neutron diffraction. We currently have at hand large crystals (ca 4 mm3) of [U(C5Me5)2(H)(mu¿H)]2, which presents us with the unique opportunity not only to measure accurately terminal U-H and bridging U-H-U distances, but also compare them with their thorium analogs. The length of a simple covalent bond between the lightest and heaviest of the naturally-occurring elements is an unknown quantity, and this study presents us with a unique opportunity to measure this key parameter.