Biodiesel + diesel fuel mixtures are commonly used in the last years especially in the transport area, in order to reduce environment pollution, and the dependency on the imported fossil fuels. Some of the basic properties of these mixtures, especially density and viscosity, strongly influence spray properties, atomization and combustion processes, engine deposits formation, engine behavior in cold weather conditions, being used as input data for predictive engine combustion models. In this study pseudo-binary biodiesel + diesel fuel and biodiesel + benzene mixtures were prepared and densities, viscosities, and refractive indices of mixtures were measured at 298.15 K. The accuracy of different mixing rules and empirical equations used to estimate these properties was evaluated. The density of the studied mixtures can be predicted with a very good accuracy using Kay's mixing rule or empirical equations obtained from regression analysis. The viscosity of biodiesel + petroleum diesel fuel can be predicted with a very good accuracy using the empirical equations. The viscosities of the two types of biodiesel mixtures are predicted with lower accuracy using Grunberg-Nissan modified equation or one equation developed for petroleum fractions. Density-refractive index and viscosity-refractive index correlations have been developed. (C) 2011 Elsevier Ltd. All rights reserved.