The NiCO molecule has been reinvestigated in solid argon, neon and mixed Ar/Ne matrices to attempt detecting a possible Ar-NiCO complex. The absorptions observed in pure neon are weakly shifted from those in argon, and the relative isotopic shifts for the two noble gas are the same. Also, adding progressively Ar to the Ne matrix demonstrates the existence of a progressive shift in the CO vibrational frequency of NiCO as the number of Ar atoms in the solvatation shell increases progressively. This greatly supports the observation of a NiCO molecule in noble gas matrices, and argues against the existence of a Ar-NiCO bound species. (c) 2006 Elsevier B.V. All rights reserved.