Skutterudite based thermoelectric unicouples are being considered for use in Advanced Radioisotope Power Systems (ARPSs) to support NASA's planetary exploration missions. For these systems, which would be much lighter than state of the art Radioisotope Thermoelectric Generators (RTGs), it is important to ensure minimal degradation in the performance of unicouples that may be caused by material sublimation. In this work, two unicouples, JAN-04 with a thin metallic coating on the legs near the hot junction to suppress antimony sublimation and SEP-03 without coating, are tested for >1000 and 3600 h, respectively. The legs in the two unicouples are of almost the same dimensions and compositions; the p-legs are made of CeFe3.5Co0.5Sb12 and Bi0.4Sb1.6Te3 segments and the n-legs are made of CoSb3 and Bi2Te2.95Se0.05 segments. SEP-03 is tested at average hot and cold junction temperatures of 961.5 +/- 22.0 and 296.3 +/- 5.7 K, respectively, in argon gas at similar to 0.068 MPa, and JAN-04 is tested at 962.8 +/- 20.5 and 294.5 +/- 3.3 K, respectively, initially in argon gas at the same pressure for similar to 26.5 h then in vacuum similar to 9.0 X 10(-7) Torr for >973.5 h. The measured open circuit voltage V-oc (240 mV) and peak electrical power (1.64 W-e) for SEP-03 at the beginning of test (BOT) are higher than those for JAN-04 (188 mV and 0.84 W-e, respectively). Although the argon gas effectively decreased the antimony loss from the legs of SEP-03, marked degradations in performance occurred. The estimated peak efficiency for SEP-03 decreased from 13.8% at BOT to 5.8% at end of test (EOT), and the peak power decreased from 1.64 W-e at BOT to 0.48 W-e at EOT, however, V-oc decreased by similar to 14%. The latter for JAN-04 decreased only by similar to 3%, the estimated peak efficiency (similar to 12%) changed very little and the peak power decreased by similar to 20%. Unlike SEP-03, the measured total and contact resistances of the legs in JAN-04 changed very little. (c) 2005 Elsevier Ltd. All rights reserved.