The five carbides R4C 5( R=Y, Gd-Ho) have been prepared by arc-melting cold-pressed pellets of the elemental components and subsequent annealing at 1050°C. They crystallize with a new orthorhombic structure type ( Pbam, Z=2), which has been determined from X-ray powder diffractometer data of Y 4C 5( a=657.35(9) pm, b=1191.8(1) pm, c=366.92(5) pm, R F=0.035 for 179 structure factors) and also refined from powder data of Tb 4C 5( a=660.8(1) pm, b=1197.3(2) pm, c=368.71(6) pm, R F=0.035 for 181 Fvalues) and Ho 4C 5( a=653.00(8) pm, b=1184.6(1) pm, c=363.80(4) pm, R F=0.036 for 171 Fvalues and 10 positional parameters each). The structure contains building blocks, which were also found in the two closely related structures of α- and β-Ho 4C 7. One-fifth of the carbon atoms are isolated from each other and coordinated octahedrally by rare earth atoms. The other carbon atoms form pairs with C-C bond distances of 133.5(15), 136(3), and 129(2) pm in Y 4C 5, Tb 4C 5, and Ho 4C 5, respectively, corresponding to C-C double bonds. Hence, all valence electrons can be accommodated in bonding R-C and C-C states according to the formula (R +3) 4(C -42) 2C -4, where the superscripts represent oxidation numbers. Nevertheless, the hydrolyses of Y 4C 5and Ho 4C 5with distilled water resulted in up to 41 wt% ethane, up to 16 wt% propane and propene, and up to 18 wt% higher hydrocarbons in addition to the expected products methane and ethylene.