This work presents viscosities and densities of systems composed of vegetable oils (or animal fat), commercial fatty acids, ethanol, and water at several temperatures. These systems are of great importance for oil/fat deacidification by liquid-liquid extraction (LLE) using aqueous ethanol as solvent, as well as for ethylic biodiesel production. Experimental data and accurate predictive models of physical properties, such as viscosity and density, are essential for process designers mainly because they affect fluid dynamics, coefficients of heat and mass transfer, and consequently column efficiency. Experimental data for viscosities were also estimated using well-known methods, such as Kay's mixing rules (simple and modified form) and GC-UNIMOD. Despite the non-ideality inherent to the mixtures studied in this work, approximately 71% of the systems showed an acceptable estimation of their viscosities using at least one of these three models, with average relative deviations (ARD) up to 10%. A simple mixing rule, based on densities of pure compounds and their mass fractions in the mixture, was used for estimating densities with ARD values not higher than 0.54%.