The present state of research on the CO dimer is reviewed. In recent years, both infrared and millimeter-wave spectra have been measured and partially assigned by the use of combination differences. A microwave-millimeter wave double resonance experiment, reported here for the first time, provides independent confirmation of these assignments and the resulting รฐCOร 2 energy level scheme. In the double resonance experiment, the OROTRON spectrometer functions as a supersensitive intra-cavity millimeter-wave detector. We update the continuing, but difficult, experimental efforts in recording the spectra, the quest for secure assignments, and the construction of a consistent and reliable energy level scheme. Although at present we have only limited knowledge of some aspects of the CO dimer, such as its geometrical structure, we have succeeded in characterizing unambiguously nine ''stacks'' of ground state energy levels with ''microwave accuracy'' ($0.1 MHz). Every energy level within a given stack exhibits the same symmetry: either A ร or A รพ . Only transitions between A รพ and A ร levels are allowed, and consequently ordinary pure rotational transitions within a stack are forbidden. Transitions between stacks can be thought of as tunneling transitions, and the separation of the lowest energy A รพ and A ร states corresponds to a value of 3:73 cm 1 for the effective ''tunneling splitting'' of the CO dimer. The stacks tend to fall into two groups, corresponding to ''isomers'' with effective inter-molecular separations of either 4.0 or 4.4
A A. The larger inter-molecular separation of the true ground state (4.4
A A) likely corresponds to a C-bonded configuration, while the low-lying รฐ0:88 cm ร1 ร excited state with the smaller separation (4.0 A A) likely displays an O-bonded geometry.