Biodiesel is a mixture of esters (usually methyl esters) of fatty acids found in the triglycerides of vegetable oils. The different fatty compounds comprising biodiesel possess different ignition properties. To investigate and potentially improve these properties, the cetane numbers of various fatty acids and esters were determined in a Constant Volume Combustion Apparatus. The cetane numbers range from 20.4 for linolenic acid to 80.1 for butyl stearate. The cetane numbers depend on the number of CH2 groups as well as the number of double bonds and other factors. Various oxygenated compounds were studied for their potential of improving the cetane numbers of fatty compounds. Several potential cetane improvers with ignition delay properties giving calculated cetane numbers over 100 were identified. The effect of these cetane improvers depended on their concentration and also on the fatty material investigated. In one case, the cetane number of the acid was increased more than that of the corresponding esters. The effect also depends on the nature of the ester. These results offer the possibility of tailoring cetane improvers to the nature of component fatty compounds in biodiesel. The cetane improving-additives are a potential route for reducing the exhaust emissions of biodiesel, for example NOx.