The molecular mobility of poly(2-hydroxyethyl acrylate) (PHEA) and of its copolymers with 2-hydroxyethyl methacrylate (H EM A) was determined by means of dynamic mechanical measurements with a torsional pendulum. The low-temperature dispersion assigned to the motion of the hydroxyethyl groups of the side chains was observed at -133°C (1 Hz) for all polymers; its intensity increased with the content of HEMA. Another secondary dispersion of PHEA detected at -95°C(1 Hz) was ascribed to the motion of the side chains including oxycarbonyl groups. With the incorporation of HEMA the temperature of the latter dispersion remained virtually unchanged, but its intensity rapidly decreased. The main transition temperature of PHEA,Ta= 9°C (1 Hz) is lower thanTaof PHEMA by approximately 94°C; the effect of the copolymer composition onTacould be described by means of a one-parameter equation. The sorption of water led to a significant decrease in theTaof PHEA and to an intensification of the secondary relaxation process.