An extensive study of the dilute solution behaviour of the carrageenans, mainly by light scattering techniques, is presented. Both kappa- and lambda-carrageenan exhibit ion-specific interaction. Normal polyelectrolyte behaviour is observed for kappa-carrageenan and a conformational transition without a change in molar mass is reported, hence, excluding a coil-to-double-helix transition under the experimental conditions used in this study. A concentration-dependent association of iota-carrageenan is observed as a function of increasing salt concentration; however all light scattering measurements extrapolate to the same molar mass at infinite dilution under conditions where the single helix prevails. The influence of counterion on chain stiffness, as judged by comparison of experimental results with theoretical predictions, is discussed in terms of the persistence length. As all results have been obtained on polydisperse samples, attempts have been made to fractionate the carrageenans. This can be successfully done for kappa- and lambda-carrageenan, but not for iota-carrageenan. The present light scattering study on iota-carrageenan provides guidelines for a move appropriate strategy to fractionate polydisperse carrageenan samples.