A testing procedure has been developed to evaluate the effectiveness of cetane improvers for improving the ignition properties of diesel fuels. Among the factors considered in developing this procedure were the availability, characterization, and toxicity of the base fuel, the ease of measurement of the data, and the sensitivity of the analysis. This particular method has several advantages over previously used methods and is suitable for obtaining fundamental information on performance trends. Performance data are reported for 2-ethylhexyl nitrate (EHN) and tetraethylene glycol dinitrate (TEGDN). The data show that inconsistencies in reported performances for these cetane improvers can be explained by different apparent activation energies for the respective ignition processes as well as the lower solubility of TEGDN.