Previous studies concerning the formation of compounds in binary systems of the type Bi2O3-MO and PbO-MO (M-Ca, Sr and Ca + Sr) have shown that the reactions which occur with the highest rate are those between Bi2O3 and CaO and between PbO and CaO. This work deals with the competition betweeen PbO and Bi2O3 in reaction with CaO, when the three above mentioned oxides coexist. DTA-TG analysis performed under non-isothermal conditions and X-ray diffraction determinations have shown that the first reaction which occurs is that between Bi2O3 and PbO. On increasing the temperature, the compound between Bi2O3 and PbO melts, leading in the presence of CaO, to the preferential formation of 2CaO . PbO2. Under non-isothermal conditions, up to 1173 K, in the ternary compositions investigated, the compound 3Bi(2)O(3) . 4CaO is also formed.