Cross-linked poly(acrylic acid) (PAA) synthesized by radical polymerization in inverse suspension is a swelling gel. The physical structure of PAA has been analyzed using low frequency chain dynamic given by the analysis of thermo stimulated currents (TSC). The alpha primary dielectric relaxation mode observed around the glass transition temperature (T-g = +38 degreesC) corresponds to the slowest dynamic. The relaxation times of the constituting processes show that it is due to a delocalized cooperative molecular mobility involving nanometric sequences of the hydrocarbon skeleton. The beta secondary dielectric relaxation mode observed at lower temperature (T-beta = -35 degreesC) corresponds to a higher frequency molecular mobility. It has been assigned to the cooperative mobility of hydrogen bonded COOH groups. In fact, the hydrogen bonded side chains behave as an hydrophilic matrix in which nanometric domains constituted by sequences of the main chain are embedded. Such a picture might explain the specific swelling properties of cross-linked PAA. (C) 2004 Elsevier B.V. All rights reserved.