The high-temperature thermoelectric properties of Sr 2 RuYO 6 and Sr 2 RuErO 6 double perovskites were evaluated and reported for the first time. These compounds show high Seebeck coefficients not only at room temperature, but also at high temperature (for Sr 2 RuYO 6 , S RT % Γ475 lV K Γ1 and S 1200K % Γ250 lV K Γ1 ; Sr 2 RuErO 6 , S RT % Γ400 lV K Γ1 and S 1200K % Γ250 lV K Γ1 ). The ntype semiconducting behaviour dominates the resistivity values. Both compounds crystallize in a monoclinic unit cell (space group P2 1 /n). The lattice parameters are a = 5.7761( 2), b = 5.7804(1), c = 8.1689(1), a = c = 90Β°and b = 90.2087(8)Β°for the Sr 2 RuYO 6 , and a = 5.7760(1), b = 5.7722(0), c = 8.1544(4), a = c = 90Β°and b = 90.2099( 7)Β°for Sr 2 RuErO 6 . The unit cell can be described approximately as p 2a p Γ p 2a p Γ 2a p , where a p is the unit cell parameter of the ideal cubic perovskite structure. High-resolution transmission electron microscopy shows an interesting three-dimensional micro-twin-domain texture where the c axis is placed in the three space directions. Structural transitions at high temperatures (T t (Sr 2 RuYO 6 ) %920 K and T t (Sr 2 RuErO 6 ) %890 K) are observed by specific heat measurement in both compounds, which are found to have a strong influence on the Seebeck coefficient and electrical conductivity.