The decrease in operation voltages, such as firing and sustain voltages, and their instability has been observed frequently and is believed to be typical of the initial operation of an alternating current plasma display panel (ac PDP). To explain this decrease, ne investigated the surface morphology, preferred crystal orientation, chemical species, and outgassing impurities of protecting MgO thin films, along with ac PDP manufacturing steps, using scanning electron microscopy, x-ray diffraction, x-ray photoelectron spectroscopy, and a quadrupole mass spectrometer (QMS). Even though no significant changes were observed in the surface morphology, a drastic reduction in carbon impurity, as well as the initial high carbon concentration in the film, was observed. Furthermore, localization of the contaminant`s in the vicinity of the surface was observed during the heat treatment. With regard to the carbon contaminant, the QMS experiments showed consistent results. In the experiment measuring the secondary electron emission coefficient, the measured value of the MgO film increased and saturated when heated in a vacuum. From these observations, it is thought that the degree of carbon contamination of the MgO surface, along with the secondary electron emission characteristics, is the dominating factor that causes the decrease in the operation voltages and their instability.