This paper presents the values of viscosity (eta) and density (rho) of binary blends of sunflower biodiesel + diesel and of fish oil biodiesel + diesel over the entire range of proportion at several temperatures. Temperatures of (293.15, 313.15, 333.15, 353.15, and 373.15) K at atmospheric pressure were studied. Excess volumes (V(E)) and deviations of viscosities (Delta eta) have been calculated from the experimental data. These thermodynamic properties are essential in the understanding of intermolecular interactions between different molecules of biodiesel, and it is important in the design of several equipments used in the biodiesel industry. The excess viscosity has been fitted using the Redlich-Kister polynomial equation. A modified Rackett's equation was used to estimate the density. The viscosity for the biodiesels was estimated using three models: Thomas, Orrick-Erbar, and modified Sastri-Rao. The critical properties were estimated by the Rackett's equation, together with the Marrero-Gani method of group contribution.