Alcohol based fuels such as ethanol and methanol that have high RON and enthalpy of vaporization have emerged to be potential additives for fossil-based fuels. The concept of using iso-stoichiometric ternary blends: gasoline (G), ethanol (E) and methanol (M) can be used to replace ethanol based fuel such as E85. This is mainly due to the biomass limit with ethanol production and the cheaper production costs associated with methanol. Ethanol and methanol with a RON 107 and 109 respectively was selected as the high RON fuel and three low octane fuels were used as base fuels. These were FACE (Fuels for Advanced Combustion Engines) fuels, more specifically FACE I, J and A, in addition primary reference fuels (PRF, iso-octane/n-heptane) were also used. A CFR engine was used to conduct the experiments. For SI combustion the CFR was operated at RON conditions that correspond to engine speed of 600 rpm and air inlet temperature of 52 oC. The engine was also operated in HCCI conditions to obtain four HCCI numbers. Blending derived cetane numbers were also obtained for compression ignition mode applications. The octane numbers corresponding to four HCCI fuel numbers and the RON were obtained for methanol concentrations of 0, 1.5, 3.5 7, 10.5 and 14% with FACE gasolines and PRFs. Also ternary blends of Gasoline-Ethanol-Methanol (GEM) with same stoichiometric air fuel ratios as that of GM were investigated. It was found that the increase of octane number of GEM blends was not linear with percentage added. This means that the blending octane number, changed from close to 200 with a small percentage down to a number closer to ethanol or methanol, 107 or 109 with larger quantities. The base fuel composition played a significant role for the blending octane number of GEM. Also base fuel octane number mattered.