[en] [M₆C₄O]I₈ (M=Y, Ho, Er, Lu) are examples of reduced rare-earth iodides with two different interstitial atoms. The compounds were syntehsized from appropriate mixtures of MI₃, M, C and M₂O₃ at 1050 C in arc-welded tanatalum containers. The X-ray structure analysis of a single crystal of [Y₉C₄O]I₈ (orthorhombic, Pmmm (Nr.59), Z=2, a=2912.7(6) pm, b=384.17(4) pm, c=1080.29(9) pm, R=0.084, R_w=0.053) exhibits octahedrally coordinated carbon in ''plane'' sections besides tetrahedrally coordinated oxygen in the ''bend'' of waved metal doubel layers. These double layers are stacked atlernately with waved iodine double layers along [001]. (orig.)