Tests were performed of skutterudite based unicouples with (MAY-04) and without (MAR-03) metallic coating on the legs near the hot junction to quantify the effect on reducing performance degradation with operation time. The p-legs in the unicouples were made of CeFe3.5Co0.5Sb12 and the n-legs of CoSb3. The MAY-04 test was performed in vacuum (similar to 9 X 10(-7) torr) for similar to 2000 h at hot and cold junction temperatures of 892.1 +/- 11.9 K and 316.1 +/- 5.5 K, respectively, while the MAR-03 test was performed in argon cover gas (0.051-0.068 MPa) at 972.61 +/- 10.0 K and 301.1 +/- 5.1 K, respectively. The argon cover gas decreased antimony loss from the legs in the MAR-03 test, but marked degradation in performance occurred over time. Conversely, the metallic coating in the MAY-04 test was very effective in reducing performance degradation of the unicouple. Because the cross sectional areas of the legs in MAY-04 were larger than those in MAR-03, the measured electrical power of the former is much higher than that of the latter, but the Beginning of Test (BOT) open circuit voltages, V-oc (204.2 mV) for both unicouples were almost the same. The peak electrical power of the MAY-04 unicouple decreased 12.35% from 1.62 W-e at BOT to 1.42 W-e, after similar to 2000 h of testing, while that of the MAR-03 unicouple decreased 25.37% from 0.67 to 0.5 W-e after 261 h of testing at the above temperatures. The estimated peak efficiency of the MAY-04 unicouple, shortly after BOT (10.65%), was only similar to 0.37% points lower than the theoretical value, calculated assuming zero side heat losses and zero contact resistance per leg. (c) 2006 Elsevier Ltd. All rights reserved.