Bi-fuel spark ignition engines, nowadays widely spread, are usually equipped with two independent injection systems, in order run the engine either with gasoline or with gaseous fuel, which can be Natural Gas (NG) or Liquefied Petroleum Gas (LPG). These gases, besides lower cost and environmental impact, are also characterized by a higher knock resistance with respect to gasoline that allows to adopt a stoichiometric proportion with air also at full load. Gasoline, on the other hand, being injected as liquid, maintains higher volumetric efficiency and hence higher power output. As a compromise solution, it could be desired to exploit the advantages of both gasoline and gas (NG or LPG), thus performing a Double-Fuel injection: as already experimented by the authors [1, 2], the addition of gaseous fuel to the gasoline/air mixture increases knocking resistance, allowing to run the engine with both “overall stoichiometric” mixture (thus lowering fuel consumption and emissions) and better spark advance (which increases engine efficiency) even at full load: the results showed high improvements in engine efficiency without noticeable power losses respect to the pure gasoline operation. Since no references have been found in literature on the Octane Number of both NG-gasoline and LPG-gasoline blends, the authors decided to experimentally determine the knock resistance increase due to gaseous fuel addition to normal air-gasoline mixtures. A wide experimental campaign has been carried out in order to evaluate the correlation between the gaseous fuel-gasoline mixture composition and its overall knock resistance measured in terms of Motor Octane Number (MON). To this purpose, a CFR engine was endowed with two independent injection systems in order to realize mixtures with different proportion between gaseous fuels and gasoline and control the overall air-fuel ratio. The experimental results presented in this paper are quite innovative and will be fundamental for future study on the simultaneous combustion of gaseous fuel and gasoline. The experimental results showed that the relationship between the mixture MON and gaseous fuel concentration in the blend is not linear and is quite different between NG-gasoline and LPG-gasoline blends.