[en] In this review, experimental results on studies of radioactive iodine, tritium and neptunium are summarized from various experimental results which the author studied in JAERI. On studies of radioactive iodine, firstly, in experiments which was simulated the reprocess of spent nuclear fuels, the following results were described such as the composition of radioactive organic iodides, the relationship between the yield of organic iodides and reaction temperatures or variation of acids, the relationship between the yield of organic iodides and chemical forms of iodine, the source of carbon for organic iodides. Then, from these experimental results, the process establishing the formation mechanism of organic iodides was fully described. On the basis of the formation mechanism, followings were also discussed on possibilities for the formation of organic iodides under nuclear reactor accidents, and the upper limit of the amount of organic iodides under the atmosphere of HTTR. On studies of tritium, three topics were concisely described on the isotopic composition of tritium gases which were commercially available, the chemical forms of tritium in various materials and the diffusivity of tritium in them, the sorption and desorption behaviors of tritium on surfaces of various materials. On the experimental results of neptunium chemistry, firstly, the relationship between the structure of neptunium compound and the ²³⁷Np Moessbauer isomer shift was discussed. For neptunium(V) compounds, a good correlation was found between the isomer shifts and the mean bond length of Np-all oxygen coordinated, however for neptunium(VI) compounds, better correlation was found between the isomer shifts and the bond length of Np-O in the neptunyl(VI) groups. Furthermore, the experimental results were described on neptunyl(VI) hydroxides in which some of them were new compounds. (author)