The high efficiency combustion of flexfuel engine depends on the precise determination of the alcohol-gasoline ratios. Presently, commercial technique available uses the Lambda probe, a post-combustion sensor to analyze the exhaust gases (after combustion). This ratio can be measured in real-time using an optical fiber sensor. In the present research, different ratios of alcohol-gasoline blends were measured at temperatures in the range 0 to 50°C, by a Fresnel reflectometer. The sensoring principle is based on the phenomenon of light reflectivity in the interface fiber-fuel. The reflected intensity is proportional to the refractive index difference between the fiber sensor and the fuel, while the refractive index of the mixture is a function of the volumetric ratio of the substances. The real time determination of the mixture ratio of alcohol-gasoline concentrations by the reflected intensity can be used to optimize the combustion efficiency of the flexfuel engine. It can be expected that this technology can reduce both, the pollutant emissions and the fuel consumption.7004 Delgado, R.C.O.B., Ara-jo, A.S., Fernantes Jr, V.J., Properties of Brazilian gasoline mixed with hydrated ethanol for flex-fuel technology (2007) Fuel Proc. Tech, 88, pp. 365-368Hammel-Smith, C., FANG, J., POWDERS, M., AABAKKEN, J., (2002) Issues Associated with the Use of Higher Ethanol Blends (E17-E24), , National Renewable Energy LaboratoryDueker, H., Friese, K., Haecker, W., (1977) Lambda-Sensor with YD2oD3-Stabilized ZroD2-Ceramic for Application in Automotive Emission Control Systems, , SAE Technical Papers(2003) A Tecnologia Total Flex, pp. 4-5. , Noticia da Oficina, ppMontanari, G., Damasceno, F., SFS Software Flexfuel Sensor, , http://www.aea.org.br/docs/ArtigosPublicacao/SFS2.pdfLima, L.C., Macedo, A.R.M., Macedo, A.R.L., Marques, E., Construção e avaliação de um monitor de combustão industrial (2007) Rev. Tecnol. Fortaleza, 28 (1), pp. 77-84Takeishi, R.T., Gusken, E., Meirelles, B.M., Raizer, K., Souza, H.G.E., Suzuki, C.K., Determination of the concentration ratios of bioethanol-gasoline mixtures for flexfuel vehicle by optical fiber sensor (2007) V International Brazil-Japan Workshop on Biofuel, Environment and New Products from Biomass, , Campinas, CD-RomKhijwania, S.K., Tiwari, V.S., A fiber optic Raman sensor for hydrocarbon detection (2007) Sensors and Actuators B, 125, pp. 563-568. , PPKim, C., Su, C., Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter (2004) Meas. Sci. Technol, 15, pp. 1683-1686Pandey, J.D., Vyas, V., Jain, P., Dubey, G.P., Tripathi, N., Dey, R., Speed of sound, viscosity and refractive index of multicomponent systems: Theoretical predictions from the properties of pure components (1999) J. of Molecular Liquids, 81, pp. 123-133Yaltkaya, S., Aydin, R., Experimental Investigation of Temperature Effect on the Refractive Index of Dye Laser Liquids (2002) Turk J. Phys, 26, pp. 41-47Turan, J., Carome, E.F., Fiber Optic Refractometer for Liquid Index of Refraction Measurements (2001) Telsiks, , 19-21 September