Accurately predicting density and viscosity is required by the production of biodiesels and biodiesel blends that can meet the required quality standards, by the modeling and simulation of injection and combustion processes, and by the design of reactors, distillation units, and separation processes. Here, the regular-solution theory is used to develop novel predictive equations for the density of biodiesels and the density and viscosity of the (biodiesel + biodiesel) and (biodiesel + petrodiesel) blends. Systematic comparisons with experimental data and with the most widely used equations are made. The new density equation provides accurate predictions for the biodiesels and the above-mentioned blends, but Kay's mixing rule cannot provide satisfactory predictions for the biodiesels. The new viscosity equation gives good predictions for the above mentioned blends; particularly, its predictions are much better than those of the Grunberg-Nissan equation, even though the Grunberg-Nissan equation uses a blend-specific parameter determined from the blend viscosity.