The dependence on salt composition of the coil-helix-aggregate transition of kappa-carrageenan has been studied experimentally and theoretically. The transition enthalpy on cooling has been measured by differential scanning calorimetry (DSC). The salt composition was varied systematically using mixtures of the four salts NaCl, NaI, CsI, and CsCl, which includes both aggregating and nonaggregating conditions. One anion or cation have been exchanged for another, keeping the total salt concentration at 0.1 M. An electrostatic model, based on the nonlinear Poisson-Boltzmann equation solved in cylindrical symmetry, was used to describe the specific binding of cesium and iodide ions to the kappa-carrageenan helix. This modeling resulted in a semiquantitative description of the variation of the ion binding and the charge density in the different salt mixtures. It was found that the model can reproduce the transition enthalpies in the nonaggregated systems very well while in the aggregated systems the model predictions deviates markedly from the experimental results. This deviation, interpreted as an aggregation enthalpy, varies with the extent of thermal hysteresis. The hysteresis occurs only when the charge density is lower than the charge density for the bare kappa-carrageenan helix (without bound ions). Another interesting observation is that the width of the DSC peak obtained on heating increases drastically when the aggregation occurs. Finally, the uniaxial gel strength was found to depend linearly on the extent of the hysteresis.