For the first time, millimeter wave absorption spectra of the van der Waals complexes He-3-CO and He-4-CO were detected in a supersonic jet. Altogether four rotational transitions of He-3-CO and six rotational transitions of He-4-CO were recorded between 110 GHz and 127 GHz with an intracavity spectrometer based on the millimeter wave generator, called OROTRON. The obtained results were included in a global fit together with previously known data enabling a more precise determination of the energy levels of the He-3-CO and He-4-CO complexes. In extremely cold, dark, and dense interstellar clouds the He-CO complex may have astrophysical relevance. (C) 2000 American Institute of Physics. [S0021-9606(00)00907-7].