In recent years, palm and jatropha biodiesels have been considered as potential renewable energy sources in Malaysia. Therefore, this experimental investigation was conducted to improve the blend of these two biodiesels (20% biodiesel blend, named P20 and J20, respectively) with the help of oxygenated additives. The comparative improvement of P20 and J20 blends with ethanol, n-butanol, or diethyl ether as additives was evaluated in terms of performance and emissions characteristics of a four-stroke single cylinder diesel engine. The final blend consisted of 80% diesel, 15% biodiesel, and 5% additive. Tests were conducted at different speeds (1200-2400 rpm) at constant full load conditions. Use of additives significantly improved brake power and brake thermal efficiency (BTE). Compared with P20 blend, the use of diethyl ether as additive increased brake power and BTE by about 4.10% and 4.4%, respectively, at 2200 rpm. A similar improvement was observed for J20. The other two additives also improved performance. Although HC emission increased slightly, all blends with additives reduced more NOx and CO emissions than P20 and J20 almost throughout the entire engine test. The use of ethanol as additive reduced CO emission by up to 40%, while the use of diethyl ether as additive reduced NOx emissions by up to 13%. The additives' oxygen content, volatility, and latent evaporation heat controlled the emissions characteristics of the blends. An analysis of the combustion chamber pressure, temperature and heat release rate of the modified blends revealed interesting features of combustion mechanism, which are indicative of the performance and emissions characteristics. This experiment reveals the potential improvement of palm and jatropha biodiesel blends with the addition of three promising additives. (C) 2014 Elsevier Ltd. All rights reserved.