Phase separation and gelation phenomena induced by addition of divalent cations in aqueous solutions of synthetic and natural polycarboxylates have been investigated by comparison of their phase diagrams. Three major types of behavior may be identified on the basis of the position and shape of the demixing line in the phase diagram, regardless of the chemical structure of the polymer. We propose a model for quantitative prediction of the demixing and sol-gel transition. The relevant parameters introduced are the neutral units, the charge density, and the cation/polyanion affinity, all of which are responsible for inter- and intramolecular complexes. Besides this general scheme, the local configuration of the backbone in natural polymers seems to promote intermolecular cross-links, leading to a large gel phase.