The effect of moisture on the mechanical relaxation processes of semiaromatic semicrystalline polyamides synthesized by a long-chain aliphatic diamine and terephthalic acid was investigated by dynamic viscoelastic analysis (DVA) and differential scanning calorimetry (DSC). Moreover, the implication of moisture with the amorphous and crystalline domains was also examined by temperature-dependent wide-angle X-ray diffraction and Fourier transform infrared spectra. The characteristics of the relaxations such as alpha, beta, gamma, and the pronounced peak shoulder appeared at 25-100 degreesC in DVA tan 8 curves were found to be strongly susceptible to the presence of moisture. With moisture evaporation, the peak shoulder of 25-100 degreesC and the beta-relaxation disappeared. The former is anticipated to originate from to the side group motion of hydrogen-bonded water, whereas the later one is from the motions of the amide-water complex units. With the disappearance of the beta-relaxation, the gamma-relaxation appeared simultaneously in much lower temperatures and ultimately coupled with the delta-relaxation. The gamma-relaxation is attributed to be associated with the molecular motion of the amide group and delta-relaxation with for the motion of the methylene units. The existence of two types of water was identified in the polymer, namely, tightly bound and loosely bound. The tightly bound water is believed to be directly connected by hydrogen bonding with the strong polar groups and the loosely bound water weakly links with those connected water making hydrogen bridges. The moisture acts as a plasticizer in the polymer matrix, which causes quite a large depression in its glass transition temperature (T-g). WAXD and FTIR studies corroborated the existence of water solely in amorphous regions, i.e., no rapport of water with the crystalline parts. (C) 2003 Wiley Periodicals, Inc.