[en] Within the homogeneous range of uranium monocarbide UB_1-xC_1+x, the crystal structures of stoichiometric UBC and of the carbon-rich solid solution UB_0.78C_1.22 have been refined from single-crystal X-ray counter data. From X-ray analysis crystal symmetry in both cases is consistent with the centrosymmetric space group Cmcm and there are no indications of superstructure formation. In contrast to the fully ordered atom arrangement revealed for stoichiometric UBC, random occupation by boron and carbon atoms is observed for the boron site in UB_0.78C_1.22. Neutron powder diffraction experiments at 9 and 295 K unambiguously revealed full occupancy by the nonmetal atoms in UB_0.78C_1.22 and prove the statistical occupation of B and C atoms in the B-sites. The boron-boron chain in Cmcm (B-B = 0.1874 nm) is dissolved into boron pairs (B-B = 0.1706 nm). The formation of C-B-B-C groups corresponds to the structure types of ThBC and Th₃B₂C₃. The magnetic behavior has been investigated in the temperature range from 4.2 K to 1000 K for UB_1-xC_1+x (UBC-type) and U_1-xM_xB₂C (ThB₂C-type for the high temperature modification and 1-UB₂C-type for the low temperature modification) with U partially substituted by Th or Sc, Lu. From magnetic susceptibilities, the alloys UB_1-xC_1+x reveal temperature independent paramagnetism with typical intermediate valence fluctuation behavior (T_SF ∼ 350 K). ThB₂C and 1-UB₂C both are temperature independent paramagnets, whereas h-UB₂C is a ferromagnet with the rather high Curie temperature T_M = 80(2) K. T_M and the saturation magnetization per U atom both successively decrease on substitution of U by Th, Sc, or Lu in UB₂C, whereas the U-moments remain practically unchanged at μ_eff(U) ∼ 1.9 μ_B. 20 refs., 8 figs., 5 tabs