Mixed ionic-electronic conductors (MIECs) based on YFeO are positioned as one of the promising cathode materials for intermediate-temperature solid oxide fuel cells (so-called protonic ceramic fuel cells or PCFCs) based on proton-conducting electrolytes. Simultaneous modification of both basic cationic sublattices of YFeO by calcium and iron was used to tailor the functionality of the obtained Y Ca Fe Сo O (x = 0.2, 0.3, 0.4 and 0.5) compounds. Following the successful synthesis of these oxides, such functional characteristics as structural, thermal, transport properties and electrochemical activity were extensively studied taking the composition (i.e. cobalt concentration, x) into account. The complex analysis carried out on the obtained data shows that Co-doping results in improved electrical conductivity and electrochemical polarisation resistance with no adverse effects on their thermal expansion, even at the highest cobalt content (x = 0.5). The performance of a single SOFC with the selected Y Ca Fe Сo O electrode confirms the prospects of Y Ca Fe Сo O phases as modernized MIECs having good thermal properties and acceptable electrochemical activity. 3 3 0.9 0.1 1–х х 3–δ 0.9 0.1 0.5 0.5 3–δ 0.9 0.1 1–х х 3–δ