Density rho and viscosity eta were measured for binary mixtures of cotton seed + babassu and soybean + babassu biodiesel over the composition range at several temperatures of (293.15, 313.15, 333.15, 353.15, and 373.15) K and atmospheric pressure. The viscosity deviation (Delta eta) was determined and fitted by a Redlich-Kister type function in terms of mass fraction (w). The Joback's method combined with Lee-Kesler and Rackett equations were used to calculate the densities of the binary mixtures of cotton seed + babassu and soybean + babassu biodiesel. The experimental and estimated density values gave almost identical values with a relative differences of less than 0.21 %. The Ceriani's group contribution method was used to predict the viscosity, with a maximum relative deviation of 31 %.