Rheology, micro-DSC and confocal microscopy were used to study the effect of potassium ion on the viscoelastic behavior, disorder-order transition and microstructure, respectively, of kappa-carrageenan in solution under different conformations at 60, 25 and 9 degrees C. At 60 and 25 degrees C the rheological behavior of 0.5% kappa-carrageenan with 0-80 mmol/dm(3) and 0-5 mmol/dm(3) KCl, respectively, was typical of viscoelastic solutions of random coiled polymers. At 9 degrees C and below a critical ionic concentration of about 7.0 mmol/dm(3), kappa-carrageenan adopted an ordered conformation in which helical structures did not aggregate and hence did not form self-supporting gels. Changes in polysaccharide stiffness were estimated from intrinsic viscosity variations as a function of ionic content. In the ordered state, the stiffness was higher than in the disordered state, whereas a liquid-like viscoelastic behavior was still exhibited. In 0.5% kappa-carrageenan at 25 degrees C, increasing KCl from 0 to 300 mmol/dm(3) produced gels of increasing rigidity. However, above 100 mmol/dm(3) such increase was marginal. Confocal images evidenced a three-dimensional network whose continuity depends on polysaccharide and salt concentrations. These observations are consistent with the rheological behavior of the self-supporting gels obtained with kappa-carrageenan concentrations in the range of 0.05-1%