When exposed to divalent counterion solutions, polyelectrolyte multilayer (PEM) films of poly(diallyldimethylammonium chloride) and sodium poly(styrenesulfonate) (NaPSS) prepared in the presence of monovalent salt, or equilibrated with such a salt, are physically cross-linked by divalent counterion incorporation, altering PEM properties significantly. The rapid cross-linking was monitored by the quartz crystal microbalance with dissipation (QCM-D) method, which finds PEM deswelling and rigidification after exposures to a low concentration of Cu(NO3)(2); at higher concentration, deswelling is countered by increased PEM uptake of the salt, which disrupts polyelectrolyte polyelectrolyte ion pairs. Divalent ion incorporation into PEMs has the character of ion exchange, and incorporated divalent ions are quickly and completely removed when presented with monovalent salt solution but not with water. While counterion cross-linking extends across the bulk of the PEM, the fraction of exchanged counterions remains low. Entropically driven binding of divalent ions to NaPSS in solution was studied for Cu(NO3)(2) and other divalent nitrate salts by isothermal titration microcalorimetry and dynamic light scattering to support the QCM-D conclusions.