Rotational spectra of the CO–H 2 S complex were studied using a cavity Fourier transform microwave spectrometer. Altogether 16 isotopomers were investigated. The tunneling splitting due to interchange of the “bonded” and “non-bonded” hydrogen (deuterium) nuclei of the H 2 S (D 2 S) subunit was observed and analyzed. In addition, the nuclear quadrupole hyperfine structures due to the quadrupolar 33 S and 17 O nuclei could be resolved and analyzed. The resulting rotational, tunneling, and hyperfine constants were used to derive structural and dynamical information about the complex.