Rheological and swelling properties of hydrogels based on pullulan crosslinked with sodium trimetaphosphate (STMP) are explained according to various polymer and crosslinking agent concentrations using P-31-nuclear magnetic resonance study. This method has allowed determining the amount of all the species present in the medium when varying both pullulan and STMP concentrations. We have clearly demonstrated with a good agreement by both P-31-NMR and rheology that a critical STMP concentration occurs which is function of pullulan concentration. This typical crosslinking agent concentration delimitates the maximum of gel structure together with the minimum of swelling.