The refractive index of carbon monoxide was derived to near 50 GPa using Infrared reflectivity. Variances in the refractive index near 5.5 GPa were related to a previously identified phase transition. A combination of refractive index and image mapping permitted extraction of the sample thickness as a function of pressure, which when fit with a third-order Birch-Murnaghan EOS, revealed the pressure-volume relationship is in good agreement with X-ray diffraction and a computationally predicted shock Hugoniot of non-reactive carbon monoxide over a limited pressure range. Published by Elsevier B.V.