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High-temperature instability of the Ca₃Co_{4&#8722;<i>y</i>}O_{9+<i>&#948;</i>} and CaMnO_{3&#8722;<i>&#948;</i>} direct p-n junction causing the formation of Ca₃Co_{2&#8722;<i>x</i>}Mn<i>_x</i>O₆ has motivated the investigation of the thermoelectric performance of this intermediate phase. Here, the thermoelectric properties comprising Seebeck coefficient, electrical conductivity, and thermal conductivity of Ca₃Co_{2&#8722;<i>x</i>}Mn<i>_x</i>O₆ with <i>x</i> = 0.05, 0.2, 0.5, 0.75, and 1 are reported. Powders of the materials were synthesized by the solid-state method, followed by conventional sintering. The material Ca₃CoMnO₆ (<i>x</i> = 1) demonstrated a large positive Seebeck coefficient of 668 &#956;V/K at 900 &#176;C, but very low electrical conductivity. Materials with compositions with <i>x</i> &lt; 1 had lower Seebeck coefficients and higher electrical conductivity, consistent with small polaron hopping with an activation energy for mobility of 44 &#177; 6 kJ/mol and where both the concentration and mobility of hole charge carriers were proportional to 1&#8722;<i>x</i>. The conductivity reached about 11 S&#183;cm^&#8722;1 at 900 &#176;C for <i>x</i> = 0.05. The material Ca₃Co_1.8Mn_0.2O₆ (<i>x</i> = 0.2) yielded a maximum <i>zT</i> of 0.021 at 900 &#176;C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca₃Co_{2&#8722;<i>x</i>}Mn<i>_x</i>O₆ layers between Ca₃Co_{4&#8722;<i>y</i>}O_{9+<i>&#948;</i>} and CaMnO_{3&#8722;<i>&#948;</i>} open for new geometries and designs of oxide-based thermoelectric generators.