We report on the preparation and characterization of interpenetrating polymer networks (IPNs) composed of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAAc) formed by a sequential method. Interpolymer interactions were examined using C-13 CP/MAS NMR and DSC methods. Evidence on the formation of a PVA-PAAc complex through hydrogen bonds between the hydroxyl groups of the PVA chains and the carbonyl group of the PAAc chains was obtained. The existence of polymer interactions between PVA and PAAc and its effect on the molecular motion of polymer chains. was further investigated by means of the analysis of the C-13 spin-lattice relaxation times in the rotating frame, T-1p(C). To elucidate the scale of the mixing in the PVA/PAAc IPNs, proton spin-lattice relaxation times in the rotating frame, T-1p(H) were also investigated. The analysis of the results reveals the compatibility between the PVA and the PAAc polymer networks at low PVA concentrations and the occurrence of phase separation at relatively high PVA concentration. (c) 2005 Elsevier Ltd. All rights reserved.