A universal equation concerning P(e)’, P(O2)" and E with no requirements to the subsequent relation of P(e)’, P(O2)’ and P(O2)" has been deduced. A "double-cell" consisting of ZrO2 (MgO) electrolyte and Cr/Cr2O3, MO/MoO2 as reference electrodes has been used for EMF measurements to determine the parameter P(e)’ at steelmaking temperatures. The influential factors to the measurement were also discussed in this paper.