A novel humectant, sodium acetylhyaluronate (AcHA), was found to have an excellent skin-softening effect on the stratum corneum. To clarify the mechanism of the skin-softening effect, the hydration behavior of AcHA was investigated by using differential scanning calorimetry (DSC). The results suggested that the amount of adsorbed water of AcHA in powder form was equal to that of HA. However, the DSC results showed that the bound water content in the stratum corneum treated with AcHA was markedly greater than that of HA-treated stratum corneum. Apparently, AcHA could enhance the intrinsic water holding capacity of stratum corneum. These findings indicate that the excellent skin-softening effect of AcHA is due to the enhancement of the bound water content in the stratum corneum.