(Received January 15, 1992; accepted after revision February 25, 1992) ABSTRACT Inoue, A. Udata, M. and Wakita, K., 1992. Smectite-to-illite conversion in natural hydrothermal systems. In: A. Meunier (Editor), Clays and Hydrosilicate Gels in Nuclear Fields. Appl. Clay Sci., 7: 131–145. Smectite-to-illite (S-to-I) conversion in natural hydrothermal systems including active and fossil geothermal fields can be characterized from the viewpoints of mode of conversion, kinetics, and mechanism and this can be related to the durability of buffer materials of high level radioactive waste. From the viewpoint of conversion, in the active geothermal field which can be used as a case study, the S-to-I conversion occurs in different ranges of present day temperature from drill hole to drill hole, which is related to the thermal history of the field up to two or three discrete illite/smectite (I/S) phases having different percents of illite layers (%I) coexist metastably at intermediate stages of the conversion. This is particularly visible in drill holes where the conversion occurs at high temperatures. The S-to-I conversion from a fossil geothermal field suggests that the greater the thermal gradient, the more the smectite illitization is facilitated. By having the average rate of entire S-to-I conversion, which was calculated from the data of K–Ar and %I in I/S, the activation energy required to complete the S-to-I conversion was estimated to be approximately 30 ± 5 kcal/mol in natural hydrothermal systems, where the S-to-I conversion was assumed as a first order reaction and the pre-exponential factor is the same as Eberl and Hower (1976). The estimated value is close to those deduced previously from laboratory experiments. This suggests that the experiments made by using specific geologic materials and conditions may be applicable to evaluate the kinetics of S-to-I conversion in hydrothermal systems from the viewpoint of an empirical equation. Nevertheless, since the S-to-I conversion in natural hydrothermal systems is in fact controlled by mechanisms of smectite dissolution and recrystallization of illite, it will be necessary in the future to construct the real reaction kinetics based on a proven mechanism.