Hydrated lime (HL) and CaO were evaluated as solid base catalysts in the transesterification of castor oil and methanol. Castor oil and the alkyl esters produced after the transesterification reaction were characterized by H-1 NMR and TLC analysis. Since full conversion of the raw materials into biodiesel (BD) was obtained with both HL and CaO, the influence of the catalyst amount, methanol: oil ratio, reaction time, and reaction temperature was studied employing HL due to the economic and process advantages of HL over CaO. The catalyst's changes throughout reaction time and after the catalyst reutilization were analyzed by X-ray analysis of the recovered mixture of HL, raw materials and products. From the Xray diffraction analysis it was corroborated that the crystalline phases after 10, 30, 60 and 120 min were Ca(OH)(2) and CaCO3 as the main and secondary phases, respectively. From the results it was clear that the catalyst's active phase after the first use was Ca(OH)(2). However, when the catalyst was reused, calcium diglyceroxide was identified by X-ray analysis as the main crystalline phase and it remained up to the third reuse. The catalyst stability was determined and the results revealed that although a conversion decrease was observed after the first reuse, the catalyst increased its activity in the second reuse maintaining a conversion up to 84% after the third one. Room-temperature biodiesel production was also investigated. In this sense, 98% conversion was achieved at 14 h of reaction. (C) 2012 Elsevier Ltd. All rights reserved.