[en] The changes of the dissolved oxygen content after deoxidation with 0.1% and 0.42% silicon were determined by fixing the oxygen as radioactive silica with radioactive silicon (silicon-31) previously charged in a sampler and by counting the radioactivity of the residue extracted from the taken sample. In both cases, the dissolved oxygen content decreased with time rapidly below the equilibrium value, reaching a minimum, then increased gradually until equilibrium was established. The mechanism of the silicon deoxidation has been discussed from the viewpoint that such change of the dissolved oxygen content is due to the non-uniform distribution of silicon built up in the melt immediately after silicon addition and to the formation of SiO sub(x) (x<2) in the region of high silicon content. The minimum oxygen content after silicon addition corresponds to the average oxygen content at the time when silicon, oxygen and deoxidation product have been distributed uniformly throughout the melt. The dissolved oxygen content is hereafter increased by the reaction SiO₂ (crucible) = Si + 2O. Also, the processes such as the growth of the deoxidation product, SiO sub(x), and the adhesion of it to the crucible wall, in other words, the deoxidation process is rate-controlled by the rate of the above reaction. SiO sub(x) is not widely different from silica in composition. (auth.)