The objective of this study was to deepen the understanding of the immobilization process of selenium (VI)contaminated soil when using immobilization agents consist of MgO and iron (II) compounds. The objective was achieved by introducing diffusion cells that allows us to physically separate soil and immobilization agent, and measuring the valence of selenium as well as the concentrations in liquid, soil, and immobilization agent phases. Experimental data showed that the addition of immobilization agents induced desorption of selenium (VI) from the contaminated soil, and the desorbed selenium (VI) was reduced into selenium (IV) by iron (II) compounds. The formed selenium (IV) was then effectively immobilized by re-sorbing on soil particles and immobilization agents. Also found was that more amount of selenium (IV) was sorbed on the immobilization agents as hydration reaction of immobilization agents proceeded. These insights obtained in this study are fundamental but important information to fully elucidate the selenium (VI) immobilization mechanisms that are required to improve the reliability of immobilization technology.