The microwave spectrum of 1-thia-closo-decaborane(9), 1-SB9H9, has been investigated in the 12-61 GHz spectral region. The molecule has C-4v symmetry. The spectra of five isotopomers have been assigned, and a precise substitution structure of the non-hydrogen atoms has been determined. It was found that the axial sulfur atom causes a substantial expansion of the 134 belt adjacent to sulfur and hence leads to a significant distortion from a regular bicapped square antiprismatic structure. The experimental work has been supplemented by high-level ab initio (MP2/6-311G**) and density functional theory calculations (B3LYP/6-311G** and B3LYP/cc-pVTZ). The agreement between the substitution structure and the two DFT calculations is very good in each case. The agreement is considerably poorer for the MP2/6-311G** calculations, particularly for the sulfur-boron bond length,