In line with the mathematical model of the MgO formation in the decarburized zone of the pitchbearing magnesite refractories in the BOF steelmaking, a simultaneous formation of MgO and CaO has been formulated. This computer model has been solved using parameters obtained from thermodynamic data and estimates based on transition rate theory. This study shows that CaO can be formed in parallel with MgO at the steelmaking temperature but its effect is so small that it may be concluded that the beneficial effect of dolomite brick derived from the CaO formation may be of no significance in practice. This result permits inferences to be drawn from the steelmaking practice in relation to the refractories performance in the BOF.