The transesterification of two different frying oils (soybean oil and a mixture of soybean and cotton seed oil) with methanol, in the presence of an alkali catalyst (NaOH), by means of low-frequency ultrasonication (24 kHz, 200 W) and mechanical stirring (600 rpm) for the production of biodiesel fuel was studied. The two different frying oils gave similar yields of isolated methyl esters both under mechanical stirring and ultrasonication. Also the physical and chemical properties of the two biodiesel fuels produced were investigated. The fuels produced were characterized by determining their density, viscosity, flash point, boiling point, cetane number, sulfur content, cloud point, pour point, cold filter plugging point, acid value, iodine value, and saponification value. From the physical and chemical properties of the two biodiesel fuels, it is concluded that these fuels have very similar properties to those of conventional diesel, except for the cetane number, which is higher, and the sulfur content of...