The carbides U₃Si₂C₂ and U₂₀Si₁₆C₃ have been prepared in well crystallized form from the elemental components by arc-melting and subsequent annealing slightly below the melting point in a high-frequency furnace. The crystal structure of U₃Si₂C₂, previously reported with the composition ‘U₃Si₂C₃’, was determined from single-crystal X-ray data and refined to a residual of R= 0.026 for 236 structure factors and 12 variable parameters. It is of a new type with tetragonal symmetry: I4/mmm, a= 357.35(5) pm, c= 1888.2(3) pm, V= 0.2411 nm³, Z= 2. The uranium and silicon atoms form a distorted body-centred cubic arrangement. The carbon atoms are on octahedral interstitial sites with five uranium and one silicon neighbours. The Si—C bond distance of 193(2) pm corresponds to a single bond. The compound may therefore be written as (3U⁴⁺)¹²⁺(2SiC^6–)^12–. The crystal structure of U₂₀Si₁₆C₃ is confirmed by a refinement from single-crystal diffractometer data: P6/mmm, a= 1037.7(2) pm, c= 800.5(1) pm, Z= 1, R= 0.036 for 199 F values and 24 variables. Magnetic susceptibility measurements indicate spin–glass behaviour and a strongly temperature-dependent paramagnetism for U₃Si₂C₂ and U₂₀Si₁₆C₃, respectively. Both compounds are metallic conductors.