A large number of light-scattering data indicate that the sulfated polysaccharide K-carrageenan undergoes an intramolecular conformational transition under suitable conditions from a semiflexible, disordered conformation to a rather rigid, helical one. By varying the parameters controlling the conformational phase diagram (i.e., by increasing the concentration of polymer or that of the supporting electrolyte, or both, or by decreasing temperature), a tertiary structure is manifested, through a progressive increase of the weight-average molar mass ((M) over bar(w)) as determined by light scattering. This can be very well explained in terms of a reversible association of stretches of such intramolecular helical structures and quantitatively described by a model of "open association" of two or more chains. The association of kappa-carrageenan was demonstrated to take place at around room temperature also in the presence of iodide ions, as long as their concentration is greater than or equal to 0.20 M. In this way the apparent inconsistency of a number of published (M) over bar(w) results can be reconciled. Additional light-scattering results indicated that the demonstrated thermodynamic tendency to reversible interchain association may lead to an irreversible aggregation of polymer chains if an improper solution preparation procedure is used.