Dynamic viscoelastic measurements were made for storage (E') and loss (E") tensile moduli of water-absorbable polymers such as nylon-6, nylon-66, poly(vinyl alcohol), ethylene-vinyl alcohol copolymers, and regenerated cellulose under the control of stepwise scanned relative humidity at constant temperature by changes in the strain frequency over a wide range. Smooth master curves of log(E') and log(E") plotted against log(frequency) were successfully obtained for all samples. The evaluated shift factors changed with the relative humidity and could be interpreted well on the basis of a concept of free volume in the amorphous region. The free volume was affected sensitively, depending on the heat-treatment condition and the types of polymers used. For nylon-66 film, the dynamic viscoelastic measurements were made at different humidities and temperatures, from which one smooth master curve was obtained. This experimental result is important in realizing the similar effect of relative humidity and temperature on the expansion of free volume of the amorphous phase: the shift factor change induced by the relative humidity change of about 30% was equivalent to the shift factor change induced by the temperature change of about 30 degreesC. That is, the time-humidity superposition principle and the time-temperature superposition principle were applicable as equivalent contributors to the mechanical property of water-absorbable polymers.