We have investigated the behavior of moisture absorption by the polyamide 6 and epoxy samples in various humid environments at constant temperature and also examined the effect of absorbed moisture on their dynamic viscoelastic properties. The moisture absorption was revealed to show the Fickian type of diffusion and to shift the dynamic viscoelastic properties of the specimen to those at lower temperature, as an effect of plasticization. Antiplasticization taken with an increase in the storage modulus was also observed in a low-temperature range below -50degreesC. The time-water content superposition was confirmed to hold at various equilibrium water contents at constant temperature for both polyamide 6 and epoxy. The relation of the shift factor, log a(H), to the equilibrium water content for polyamide 6 has a form similar to WLF equation of time-temperature superposition, whereas the log a(H) for epoxy does not have such a form. (C) 2004 Wiley Periodicals, Inc.