In recent years, the acceptance of fatty acid methyl esters (biodiesel) as an alternative fuel has rapidly grown in Spain. The raw materials to produce biodiesel in this country include traditional seed oils (sunflower and rapeseed), alternative seed oils (Brassica carinata), genetically modified vegetable oils (high oleic sunflower), and used frying oils. In this study, the above vegetable oils with free fatty acid content from 0.02 to 6.47% were transesterified with methanol using potassium hydroxide as a catalyst in a batch-stirred reactor. Biodiesel yield and ester content were independent of the type of vegetable oil, but both decreased when the vegetable oil acid value increased due to the neutralization of the free fatty acid content in the oil. Yield losses were also due to triglyceride saponification and methyl ester dissolution in glycerol, according to the material balance of the process. On the other hand, the stoichometric potassium hydroxide to neutralize the free fatty acids was added to the amount of potassium hydroxide used as a catalyst for the vegetable oils with the highest proportion of free fatty acid. In every case, biodiesel met the glyceride concentration specifications, but the process gave much lower yields. The viscosity, peroxide value, and acid value were within EU specifications for the methyl ester evaluated. The iodine value was above the EU specification for the methyl esters from sunflower oil and low erucic B. carinata oil.