Abstract The rotational spectra of the van der Waals complex of carbonyl sulfide loosely bound to two helium atoms were investigated using a cavity Fourier transform microwave spectrometer and a microwave–microwave double resonance spectrometer. The lowest rotational transitions of seven isotopomers, namely those containing the 16O 12C 32S, 16O 12C 34S, 16O 13C 32S, 16O 12C 33S, 18O 13C 32S, 16O 13C 34S, and 17O 13C 32S subunits, were studied. Hyperfine structures due to the quadrupolar 33S and 17O nuclei were also observed and analyzed. The search for higher J transitions was not successful and possible causes are discussed. Information about the OCS–He 2 cluster inferred from the measurements is presented and discussed.