Perturbation viscometry has been used to measure the viscosity of carbon monoxide-helium mixtures relative to the viscosity of carbon monoxide for points covering the full composition range at 25 degrees C and 0.11 MPa (abs). The viscosity versus composition curve is shown to have a maximum at around 9 % carbon monoxide. The results obtained agree, to within the uncertainty of the curve predictions from Chapman-Enskog using the parameters determined by the theory of corresponding states.