The relative permittivity of soybean biodiesel/soybean oil blends was determined in the full range of composition, at temperatures between 298 K and 343 K. Measurements were carried out at frequencies between 1 kHz and 100 kHz, using a broadband cell. The relative RMS uncertainty of the dielectric measurements was below 1%. For all the studied samples, the relative permittivity decreases with temperature and increases with biodiesel content, in both cases linearly. From the experimental results, a model was developed to estimate the permittivity of the blends as a function of composition and temperature. The RMS uncertainty of the estimation is less than +/- 0.01. Furthermore, the blend composition was modeled as a function of permittivity and temperature. The RMS uncertainty of the estimation was below 9% for temperatures between 298 K and 313 K.