Viscosity is an important property of fuels for internal combustion engines and has normative values in quality standards for commercial fuels used in compression ignition engines. Elaborating reliable models to estimate viscosity of biodiesel-diesel-bioethanol blends is useful to blend fuels with high biofuel content that comply with the values of viscosity imposed in quality standards and to model and simulate injection and combustion processes for such fuels. For predicting viscosity of rapeseed oil biodiesel-diesel-bioethanol blends, 15 ternary mixtures were prepared, having a maximum amount of biofuel of 30% v/v. The concentration of biodiesel and bioethanol varies between 5% and 25%, in increments of 5%. Their density and viscosity were determined in nine steps in the temperature range 273.15-343.15 K. Using experimental data, a number of models to estimate the viscosity of liquid substances were evaluated, including Kay's rule, simple and weighted semi-logarithmic model, and Refutas model. A simple additive model was proposed, named Kay's weighted rule. The models used to describe viscosity of biodiesel-diesel-ethanol blends were critically evaluated and compared on the basis of the correlation coefficient and the percent relative deviation, demonstrating that the model proposed here provides the most accurate estimates for viscosity of biodiesel-diesel-bioethanol blends.