Waste edible oil (WEO) is the waste product of cook­ing or frying foods. The disposal of WEO is difficult and thus the use of WEO as a biofuel would both alleviate the problem of disposal in addition to providing a renewable source of biodiesel. WEO has a high volume of FFAs, 0.5—15% in comparison with the 0.5% content of refined virgin vegetable oil, which cannot be con­verted to biodiesel using an alkaline catalyst as the FFAs undergo a saponification reaction with the catalyst thus reducing efficiency and yield (Knothe et al., 2005). The problem may be overcome by using a supercritical methanol transesterification for the transesterification process rather than an alkaline catalyst (Kusdiana and Saka, 2004). The volume of WEOs available is quite high with approximately 1 million tons produced in Europe each year while 10 million tons are produced annually in the United States (Gui et al., 2008). WEO is available two to three times cheaper than virgin vegetable oils (Phan and Phan, 2008) and the high volume of WEO available means it a viable method for biodiesel production. WEO has a higher estimated energy balance than rapeseed and soybean of 5.8; however, the value is lower than that of palm oil at 9.5 (Food and Agriculture Organization, 2008).