The heterogeneous structure of poly (vinyl alcohol) -poly (sodium acrylate) composite hydrogels (PVA-PAA gels), prepared by repeatedly freezing and thawing mixed aqueous solutions of PVA and PAA, has been studied using pulsed NMR. The free induction decay (FID) signals of the transverse magnetization for the PVA-PAA gels were measured by both Carr-Purcell-Meiboom-Gill and solid-echo pulse sequences. The FID signal was decomposed into two or three decaying signals with various spin-spin relaxation times, based upon differences in the molecular mobility that reflected structural heterogeneity. The structural heterogeneity of the PVA-PAA gel is quite different from that of PVA gel free of PAA. We have made observations on "crosslinked" domains in the PVA-PAA gels. They are formed by PVA solid chains, probably phase-separated in the freezing-thawing process. The formation of crosslinked domains is influenced by the number of freezing-thawing cycles.