To discern the most important parameter determining the concentration of paramagnetic centers, the effects of chemical treatments before and/or after oxidation, the type of furnaces, the cooling rate after oxidation, and oxidation temperature were examined. The effects of chemical treatments for cleaning on Pb and E’ centers before and/or after oxidation were negligible. Two types of furnaces were prepared: One where contamination by the outer air could be prevented and one where it could not be prevented, called an air‐removed furnace and a conventional furnace, respectively. The concentration of Pb and E’ centers at the Si/SiO2 interface fabricated in a conventional furnace was, however, drastically smaller than that fabricated in an air‐removed furnace. The cooling rate after oxidation did not determine the concentration of E’ centers but affected Pb centers. The oxidation temperature also determined the concentration of both Pb and E’ centers. It was found that the parameter most effective in determining the concentration of paramagnetic centers was the type of furnace, whether there was contamination from air or not. The paramagnetic centers (such as Pb and E’ centers) react with some element(s) in the air. It is expected that such elements are not N2 gas but hydrogen‐related impurities in the air, perhaps H2O.