Abstract The cationic substitution of Cu by Co in the Bi-based superconductor was studied in order to determine the optimal sintering conditions to produce single-phase materials. X-ray diffraction experiments combined with differential thermal analysis (DTA) and electrical resistivity were utilized as the diagnostic techniques. The phase dependence on the starting stoichiometries as well as the atmosphere and temperature employed during the sintering process were established. Samples in the Bi-Sr-Co-O system were heat treated under both air and nitrogen atmospheres in the temperature range 800 < T < 900° C. The formation of Bi 2Sr 2CoO y is favored at higher temperatures and in reducing atmosphere, while the Bi 2Sr 3Co 2O y compound occurs for lower temperatures and in a partially oxidized ambient. The Co valence state in these compounds and the facility to grow single crystals are discussed.