A series of rare earth/alkali oxide carbonates LnKO(CO3) with Ln=La, Pr, Nd, Sm, Eu has been prepared by the low temperature synthesis from hydroxide/fonnate melts. Tile structure of these compounds corresponds with that of T* type oxocuprates (Ln(A)Ln(B))(2)CuO4. They are characterized by complete replacement of the transition metal/oxygen layers with carbonate layer units and represent thus the first end members of layered oxocuprate phases with partial replacement of the Cu/O layers by CO3 sheets.