In the last years new and stricter pollutant emission regulations together with raised cost of conventional fuels resulted in an increased use of gaseous fuels, such as Natural Gas (NG) or Liquefied Petroleum Gas (LPG), for passenger vehicles. Bi-fuel engines represent a transition phase product, allowing to run either with gasoline or with gas, and for this reason are equipped with two separate injection systems. When operating at high loads with gasoline, however, these engines require rich mixtures and retarded combustions in order to prevent from dangerous knocking phenomena: this causes high hydrocarbon (HC) and carbon monoxide (CO) emissions together with high fuel consumption. With the aim to exploit the high knock resistance of NG maintaining the good performances of gasoline, the authors experienced, in a previous work , the simultaneous combustion of NG-gasoline mixtures on a series production Spark Ignition (SI) engine, obtaining, with respect to pure gasoline operation, strong reduction in pollutant emissions, noticeable efficiency increase and no significant power losses. In order to ascertain the knock resistance increase due to the addition of natural gas to gasoline, and given the total absence of such information in the scientific literature, the authors decided to carry out a dedicated experimental campaign: a standard Cooperative Fuel Research (CFR) engine, properly upgraded to inject both fuels in the intake duct, has been employed to determine the Motor Octane Number (MON) of several fuel mixtures following the ASTM Standard D2700. The results showed a non-linear relation between mixture knock resistance and natural gas concentration, correctly interpolated by a polynomial law. The correlation found can be usefully implemented in knock onset prediction sub-models for thermodynamic engine simulation, or used in future works involving the simultaneous combustion of natural gas and gasoline in a SI engine.