DC conduction for non-oriented iodine-doped nylon 6 (iodine-nylon 6 complex) is investigated in environments of some relative humidity (RH) at room temperature after ageing in vacuum. While the complex sample aged in vacuum shows little DC conductivity, it is recovered immediately by moist environment, and conductivity responds to change in RH in time of a few seconds or minutes and is stabilized under equilibrium in the humidity. On the other hand, some complex samples without ageing in vacuum show hysteresis by applying DC field continuously. Such a response of conduction to RH is varied by cations, which were diffused together with polyiodide in the complex. It is suggested that the response to RH can be attributed to the mobility of the cation in the amorphous phase of the complex; DC conduction in the complex is an electrochemical process that is attributed to the cations in the amorphous phase of the complex and to its structure. The structure of the amorphous complex responds to RH or water molecules in the environment. (C) 2000 Elsevier Science S.A. All rights reserved.