Oppositely charged polyelectrolytes in solution spontaneously associate into hydrated complexes or coacervates, PECs. The morphology, stability, and properties of PECs depend strongly on their ion content, which moderates the "sticky" reversible interactions between Pol(+) and Pol(-) oppositely charged repeat units. Here, it is shown that the distribution of ions between a PEC and the aqueous solution in which it is immersed is accurately predicted by the Donnan equilibrium. For ideal, stoichiometric mixing of polyelectrolytes, corresponding to an enthalpy of complexation Delta H-PEC -> 0, the salt, MA, concentration inside the PEC, [MA](PEC), is equal to the solution salt concentration, [MA](s). Isothermal calorimetry measurements along a Hofmeister series show that if mixing is exothermic, [MA](PEC) < [MA](s), while for endothermic association of Pol(+) and Pol(-), [MA](PEC) > [MA](s). A set of simple self-consistent expressions illustrate PEC salt response without consideration of net Coulombic or electrostatic forces between charged species. Delta H-PEC exactly predicts deviations from ideal Donnan equilibria, which are connected to the equilibria between associated or intrinsic pairs of Pol(+)Pol(-) and extrinsic Pol(+)A(-) and Pol(-)M(+) pairs, where counterions compensate polyelectrolyte charges. The equilibrium constant K-pair for Pol(+)Pol(-) pair formation is shown to be proportional to the volume charge density of the hydrated, ion-free complex. K-pair may also be used to estimate the critical salt concentration at which polyelectrolytes completely dissociate.