The methyl esters of vegetable oils and animal fats, known as biodiesel, are receiving increasing attention as an alternative fuel for diesel engines. Although the production of biodiesel involves a relatively simple chemical process, there is potential for various contaminants to be present in the fuel. These contaminants include water, free glycerin, bound glycerin, alcohol, free fatty acids, soaps, catalyst, unsaponifiable matter and the products of oxidation. As interest in this fuel grows, quality standards and specifications are being developed. These standards place limits on the amounts of contaminants that may be present in biodiesel. The objective of this project was to develop a database of property data to provide a basis for setting realistic specification values for biodiesel. Small amounts of these various contaminants were added to biodresel and their impact on the properties and performance of the biodiesel was measured. The results generally support the levels given in a standard proposed by the National Biodiesel Board that is currently being considered by the American Society for Testing and Materials. However, the study identifies water contamination, bound glycerin, and oxidation as three areas of concern. Biodiesel can absorb up to 40 times more water than conventional diesel fuel. High levels of bound glycerin can cause crystallization and increased viscosity. Oxidation can produce chemical compounds that improve cetane number but also increase the acidity of the fuel. These areas need additional research.