In this study, the Transesterification Double Step Process (TDSP) for the production of biodiesel from vegetable oil was modified to yield a shorter reaction time and products with improved quality. TDSP consists in a two step transesterification procedure which starts with a basic catalysis, followed by an acidic catalysis. The process modifications included a reduction in the concentration of catalysts, a reduction in the reaction time of the first step and the direct mixing of methanol/acid solution, without cooling the system between the first and second step. A comparison between washed and unwashed biodiesel demonstrates that the final washing and drying procedure is necessary for satisfactory results. The products were analyzed by H-1-NMR and nineteen different biodiesel analyses specific for international quality certification. The modified procedure resulted in a high conversion index (97% for waste cooking oil and soybean oil and 98% for linseed oil) and high yield (87 +/- 5% for waste cooking oil, 92 +/- 3% for soybean and 93 +/- 3% for linseed oil). The biodiesel produced by the modified TDSP met ASTM, EN ISO and ABNT standards before the addition of stabilizer. (C) 2011 Elsevier B.V. All rights reserved.