Biodiesel is a renewable fuel comprised of fatty acid methyl esters (FAME) derived from vegetable oils or animal fats. Comparisons between biodiesel and petroleum-based diesel have shown biodiesel to be effective in reducing exhaust emissions of carbon monoxide, hydrocarbons, particulate matter, and Sulfur dioxide. While there are advantages of biodiesel over the traditional petroleum based diesel, biodiesel commercialization is limited by production cost that is dominated by the price of the feedstock (soybean oil). Yellow grease has the potential to be an effective feedstock with lower cost, but the chemical composition of these oils is variable depending on the source of collection and differs from that of virgin oil due to the presence of free fatty acids (FFA). Esterification has beer) previously demonstrated to reduce the FFA levels of YG, however, large quantities of methanol were required to drive the reaction to high yield. Methanol usage for processing and FFA content are the main factors affecting the economics of FAME production from YG. In this study, the relationship between composition and process variables was systematically studied. The effect of FFA ranging from 2% to 32% (w/w) was studied at three different molar ratios of methanol to FFA (4.5:1, 9:1, 18:1) and was found to have a non-linear relationship. Data obtained from this full factorial screening was used to develop a predictive statistical model to forecast the conversion based on initial FFA level and proportion of alcohol applied for esterification. (C) 2008 Elsevier Ltd. All rights reserved.