In this paper, the assignment of the 'H and I3C NMR spectra of poly(ga1acturonic acid)s is proposed; the 4C, chair conformation of the monomeric unit in poly(ga1acturonic acid) is confirmed and the composition of the anomeric mixture at the reducing end (~-D)/@-D) = 35 : 65 mol/mol independent of the degree of polymerization DP (2 < DP < 5) is given. It is demonstrated on oligomers that both 13C and 'H chemical shifts and coupling constants are nearly unaffected by exchanging Na counterions with Ca; the osmotic coefficient of Ca depends on the DP just as predicted by an electrostatic mechanism. The intrinsic viscosity of the polymer increases and the intensity of I3C NMR signals decreases suddenly for 0,3 equivalents Ca2+ per carboxylic site. These results confirm a multichain association followed by a rigid gel formation; the crosslink is proposed to correspond to a cooperative "egg-box" mechanism on chain segments randomly distributed in the gel.