The molecular mobility of poly(acrylic acid) (PAAc) and its copolymers with 2-hydroxyethyl methacrylate (HEMA) was investigated by means of dynamic mechanical measurements carried out with a freely oscillating torsional pendulum (∼1 Hz). An increase in the content of AAc in the copolymers leads to reduction and eventually to complete disappearance of both the low-temperature (−135°C) and secondary (28°C) dispersion, while the glass transition temperature passes through slight maximum. In the temperature range under investigation (from −196 to 140°), PAAc exhibits only the glass transition at Ca 108°C (1 Hz). The sorbed water markedly decreases the glass transition temperature of PAAc and gives rise to a secondary dispersion near −80°C; with increasing water content, the dispersion is shifted towards lower temperatures, and its intensity increases. The effect of AAc on the temperature and intensity of the low-temperature dispersion of PHEMA was compared with that of six other comonomers; it was found that the intensity of dispersion increased approximately linearly with the volume fraction of the HEMA component, while the temperature of dispersion did not seem to be readily correlated with the comonomer composition.