The novel carbidonitridosilicates Ho₂[Si₄N₆C] and Tb₂[Si₄N₆C] were obtained by the reaction of the respective lanthanoid metal with carbon and Si(NH)₂ in a radiofrequency furnace at the temperature of 1700 °C. According to the single-crystal structure analysis of Ho₂[Si₄N₆C] (P2₁/c, Z = 4, a = 593.14(1), b = 989.74(1), c = 1188.59(2) pm, β = 119.58(4)°, R1 = 0.0355, wR2 = 0.0879, 2187 F² values, 119 parameters) the compound contains a condensed network of corner-sharing star like [C(SiN₃)₄] units. The holmium ions are situated in channels along [100]. The UV–VIS absorption spectrum of Ho₂[Si₄N₆C] shows the typical Ho³⁺ absorption bands. The spectroscopic results show that the 4f states remain almost unaffected by the coordination sphere and thus it is impossible to distinguish between the two crystallographic sites of Ho³⁺ in the UV–VIS spectrum. Magnetic susceptibility measurements of Tb₂[Si₄N₆C] and Ho₂[Si₄N₆C] show Curie–Weiss behaviour above 150 K with experimental magnetic moments of 9.57(6) μ_B/Tb and 10.27(4) μ_B/Ho. The Weiss constants are −15(1) K and −11(1) K for the terbium and holmium compounds, respectively. Down to 2 K no magnetic ordering could be detected. The magnetization curves at 2 K show an increase of the magnetization with increasing flux density, indicating partial parallel spin alignments. At 5 T the magnetizations reach values of 4.15(5) μ_B/Tb and 4.75(5) μ_B/Ho, respectively.