The benefits of blending ethanol into gasoline fuel are well established. Ethanol’s high latent heat of vaporisation and chemical auto-ignition resistance combine in producing significant knock resistance, enabling higher compression ratio and/or higher charge boosting. Its high flame speed characteristics result in shorter burn durations. Its high knock resistance and rapid burning enable ignition phasing optimisation. These factors all improve the efficiency of spark ignition (SI) engines.Current “flex-fuel” vehicles are designed to operate on both conventional gasoline as well as blends containing higher volumes of ethanol and/or methanol, the former being commonly known as E85. The American Society for Testing and Materials ASTM D5798 specification for ethanol fuel blends was adapted in 2011 to prescribe a minimum ethanol content of 51 % with the remainder able to consist of low octane blending streams. This opens the door for fuel manufacturers to include naphtha streams previously not regarded as suitable for spark ignition engines. In this study Low Temperature Fischer-Tropsch (LTFT) Gas to Liquids (GTL) naphtha, a low octane product of a LTFT GTL refinery, was investigated for suitability of use in oxygenated gasoline fuel blends.Fuel blends consisting of a mid-grade gasoline, GTL naphtha and ethanol were analysed to compare their octane ratings in relation to gasoline specifications.The performance of GTL naphtha and ethanol blends (E50 and E85) was compared to petroleum-derived gasoline as well as a High Temperature Fischer-Tropsch (HTFT) Coal to Liquid (CTL) derived synthetic commercial 95 RON gasoline by means of spark advance sweeps conducted on a charge boosted single cylinder Gasoline Direct Injection (GDI) research engine. While the E85 blend exhibited completely knock free operation, it was shown that the E50 blend resulted in similar performance to the commercial gasoline as a consequence of comparable partially knock limited spark advance (KLSA) behaviour.The study derived blending rules for the simultaneous use of GTL naphtha and ethanol in order to maintain the octane quality of the base gasoline.