The result of the investigation on methyl esters obtained on the basis of heated refined sunflower oil and used frying oils are given in the paper. Transesterification reaction conditions that affect yield and purity of the product esters including oil quality, molar ratio of methanol to vegetable oil, type and concentration of alkaline catalyst, temperature and reaction time were examined. The methanolysis of different oils at 25degreesC with 0.5-1.5% potassium hydroxide or sodium hydroxide were studied. The effect of molar ratio 4.5:1, 6:1 and 9:1 on ester yield and its quality were investigated. With 1% potassium hydroxide, temperature at 25degreesC, molar ratio 6:1 and 30 min, all investigated oils were sufficiently transesterified and could be used as fuel in diesel engines. (C) 2002 Published by Elsevier Science B.V.