Anion photoelectron spectra are reported for the iodide-carbon monoxide clusters, with supporting ab initio calculations for the 1:1 dimer anion and neutral complexes. A C-s minimum geometry is predicted for the anion complex, while for the neutral complex two linear van der Waals minima are predicted differing in the attachment point of the iodine, that is, I center dot center dot center dot CO and I center dot center dot center dot OC. The predicted adiabatic photodetachment energy agrees well with the experimental spectrum. The photoelectron spectra feature a vibrational progression in the CO stretching mode, which becomes more pronounced for the larger clusters.