Non-oriented nylon 6 which was doped with iodine in an I-2-KI aqueous solution (iodine-nylon 6 complex) was aged in environments with different relative humidities (RH). Relative increase in mass (Deltam) of the complex was measured in these RH approximate to 0-97% environments at room temperature after aging in vacuum. The Deltam was investigated under equilibrium on aging in each RH environment. Change of Deltam in the process of increasing RH differed from that in the reverse process of decreasing RH. The Deltam decreased through aging in an RH approximate to 97% environment for about 1 or 2 months. This behavior suggests that another diffusion process is induced by swelling with moisture; it can be attributed to the process in the crystal phase of the complex which is much slower than that in the amorphous region. Polyiodides and cations adsorbed not only in the amorphous phase: but also in the crystal phase of the complex are also considered to be released when the complex has become swollen. The polyiodides which quickly diffused into the crystal phase are gradually released through the swollen amorphous phase in the high RH environment. (C) 2001 Elsevier Science B.V. All rights reserved.