The rotational spectrum of the new reactive triatomic molecule chloro(sulphido)boron, C1B=S, produced by the high-temperature reaction of gaseous dichloro disulphide, Cl2S2, and crystalline boron at ca. 1000°C was studied by microwave spectroscopy between 26.5 and 40 GHz. Ground state rotational constants have been obtained for 11 of the 12 isotopic variants involving 35C1, 37C1, 11B, 10B, 32S, 33S, and 34S; the isotopic shifts for the ground state lines of 35C110B 34S from those of 35Cl11B34S are too small and they are overlapped by the 11B species. The abundance of rotational constant data has allowed a detailed comparison of various structure determination procedures to be made. The substitution method yields an extremely consistent Cl-S distance of 3.28715 ± 0.00005 Å. Application of the first moment condition allows the B atom, which lies close to the center of mass, to be quite accurately located. The resulting bond lengths are r(ClB) = 1.681 ± 0.001 Å and r(B=S) = 1.606 ± 0.001 Å. The more important derived spectroscopic parameters are: (Table Presented). Copyright © 1980 by Academic Press, Inc.