Some important properties of biodiesel such as viscosity, melting point, thermal stability, and cetane index can be directly related to the chemical composition of the biomass source used. However, the viscosity of castor oil ethyl ester (COEE) is about four times greater than the biodiesel viscosity specification, and this fact restricts the uses of this biofuel. The main goal of this work is to investigate the physical–chemical properties of COEE in ethanol blends and present an option to decrease the castor oil biodiesel viscosity specification. COEE viscosity is 14.413 mm2/s. The results show an evident decrease in biodiesel viscosity with the addition of ethanol. Samples with 30, 40, and 50 vol % of ethanol present the following viscosities 5.316, 4.044, and 3.136 mm2/s, respectively. These results are in agreement with Brazilian National Agency of Petroleum, Natural gas and Biodiesel (ANP), and European (EN 14214) specifications. Three correlations that describe the decrease in viscosity and density with ethanol concentration, and the variation of viscosity with the density in COEE ethanol blends were obtained. The behavior of COEE ethanol blends with diesel also were analyzed. The increase in solubility of ethanol in commercial diesel was verified because COEE acts as a cosolvent in an ethanol–diesel mixture.