The SOIR instrument on board the ESA Venus Express mission has been operational since the insertion of the satellite around Venus in 2006. Since then, it has delivered high quality spectra of the atmosphere of Venus. Spectra are recorded in the IR spectral region (2.2–4.3 μm) using the solar occultation geometry and give access to a vast number of ro-vibrational lines and bands of several key species of the atmosphere of Venus. Here we present the retrieval strategy applied to obtain high quality vertical profiles of carbon monoxide (CO) densities and volume mixing ratios (vmr), spanning the 65–150 km altitude range. We discuss the methodology used to derive the profiles and the validation process implemented to ensure the quality and reproducibility of the results. Influence of ancillary data, such as temperature, is discussed. High variability of CO densities and vmr is observed in relatively short term periods. Correlation between CO and CO2 densities, as well as between CO and temperature above 110 km, corroborates that the major process at those altitudes is the photodissociation of CO2 into CO.