Knowledge of the succession of phases in dependence on composition and temperature is essential in order to control the formation of pure superconducting thallium-based cuprates (Tl/Pb)(m)Sr2Can-1CunOx. Mixtures (Sr/Ba)O-CaO-CuO are used as precursors in their elaboration. In the first stage of our investigation, the ternary system SrO-CaO-CuO has been reinvestigated at 950 degrees C, in order to determine precisely the phase relationships in the ternary subsystem. In the second stage, the study will be extended to the quaternary BaO-SrO-CaO-CuO system. The present paper reports the results concerning the polyphased fields in the ternary system under isothermal conditions at 950 degrees C under 1 bar of oxygen. A comparison with literature data shows that new domains are observed. Solid phases mentioned in the literature are confirmed, but the observed three phases equilibria are not the same.