In this work, the preliminary results obtained in the transesterification of castor-oil with methanol in the presence of ultrasonic irradiations, are presented. The fundamental objective was to study the acceleration that the ultrasound causes in the reaction rate. The effect of different operating variables, such as frequency and supplied power dissipation, catalyst (KOH) concentration and methanol oil molar ratio were also investigated. The evolution of the process was followed by gas chromatography, determining the concentration of the methyl esters at different reaction times. The biodiesel was characterized by its density, viscosity, saponification value, iodine value, acidity index, water content, flash point, combustion point, cetane index and cold filter plugging point (CFPP), according to ISO norms. Between these properties, viscosity, the number of cetane and the CFPP do not attain the values established by the standard EN 14214. These circumstances would force to use the castor biodiesel oil mixed with diesel fuel oil, or to add additives in order that the previous properties reach the specified values. High methyl ester yield and fast reaction rate could be obtained, for castor oil transesterification, under ultrasonic conditions, even if the reaction temperature was relatively low. In general, the variables of operation tested exercise a positive effect on the reaction rate, but the obtained final yield (equilibrium) is very similar in all cases. (c) 2012 Elsevier B.V. All rights reserved.