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The refractive index of blends of soybean biodiesel with low sulfur diesel and also with ultra-low sulfur dieselwas determined in the full composition range. Measurements were carried out acording to ASTM D1218/12standard at 589 nm with an uncertainty of 2.10−4. The temperature ranges were from 288 K to 328 K for blendswith low sulfur diesel and from 293 K to 323 K for blends with ultra-low sulfur diesel.The experimental results of refractive index as a function of temperature and composition were satisfactorilyfitted to linear models with an uncertainty lower than 5.10−4 for blends with low sulfur diesel and 4.10−4 forultra-low sulfur diesel blends.The density of the blends was measured according to the ASTM D1298/12 standard in the same temperaturerange and the agreement with models available in the literature was satisfactorily checked. The effective polarizabilityof the blends was defined from the Lorentz-Lorenz equation using refractive index and density values.The effective polarizability for low sulfur diesel and ultra-low sulfur diesel were (0.3307 ± 0.0005) cm3/g and(0.3327 ± 0.0005) cm3/g respectively, and independent of temperature. These results are in good agreementwith the published values for pure hydrocarbons and crude oils. For soybean biodiesel, a value of(0.3110 ± 0.0002) cm3/g was obtained. It was also found that the effective polarizability of soybean biodieseldieselblends follows a linear dependence with composition. From our results, it also follows that the effectivepolarizability is independent of temperature, within the studied range.From the values of effective polarizability of the pure fuels, together with the blend density at each temperature,it is then possible to estimate the refractive index of biodiesel-diesel blends in the full range of compositionand studied temperatures, with an RMS difference below 6.10−4.