A frozen solution of [U(eta(7)-C7H7)(2)](-) is Studied by EPR and angle-selected ENDOR spectroscopy. EPR confirms the anticipated f(1) configuration of this compound. Analysis of the g tensor indicates that the ground state molecular orbital is principally made of 5f(pi) (51%) and 5f(sigma) (39%) uranium orbitals. This composition results from the combined effect of the spin-orbit interaction and the strong f(delta)-ligand interactions. The small admixture of 5f(phi) (9%) uranium orbitals could result from a low-symmetry effect imposed by the solid-state packing forces at low temperature. Starting from the ground-state orbital, the f-level ordering: consistent with the g tensor is found to be (f(pi),f(sigma)) < f(phi) < f(pi) < f(phi), (f(sigma),f(pi)) < f(delta) (antibonding). Proton ENDOR spectroscopy shows that there is a positive spin density p(pi) greater than or equal to 4 x 10(-2) in 2p(pi) carbon orbitals. which manifests the covalent character of the ground-state orbital.