Novel composite materials based on La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) and a binary eutectic carbonates (52 mol% Li2CO3:48 mol% Na2CO3) are potential electrolytes for low-temperature solid oxide fuel cells (LTSOFCs) operating at 400-600 degrees C. However, thermal stability of the LSGM-(Li/Na)(2)CO3 composites remains in doubt due to the molten state of the carbonates at elevated temperature. In this paper, XRD, SEM, TGA and EIS were employed for thermal ageing and cycling studies of the LSGM-(Li/Na)(2)CO3 composites. XRD and SEM results showed that ageing induced a slight effect on the structure and morphology of the composites. TGA and EIS results indicated that the composites had a good stability during cycling. The LSGM-20 wt% (Li/Na)(2)CO3 sample showed a relatively stable conductivity (7-9 x 10(-2) S cm(-1)) during a 650 h measurement under air at 600 degrees C. Single cell based on the composite electrolytes was reported for the first time, a maximum power density of 617 W cm(-2) and the open circuit voltage (OCV) of 1.01 V were achieved at 600 degrees C for the composite containing 20 wt% carbonates. The notable thermal stability together with fairly high performance emphasize the promise of LSGM-(Li/Na)(2)CO3 composite electrolytes for long-term LTSOFCs. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.