The properties of polyelectrolyte complexes, PECs, made from blended polycations, Pol(+), and polyanions, Pol(-), are routinely studied under conditions where they are at least partially swollen with water. Water plasticizes PECs, transforming them from an intractable, glassy, and brittle state when dry to tough and viscoelastic when wet. In the present work the supreme efficiency of water, compared to other solvents on a polarity scale, in swelling a PEC is illustrated. Using a PEC of poly(diallyldimethylammonium) and poly(styrenesulfonate) with precisely determined density, we show that swelling tracks a Dimroth-Reichardt polarity scale until the molecular volume exceeds, similar to 50 angstrom(3), whereupon the degree of swelling drops sharply. Long-term (>1 year) swelling of this PEC in pure water reveals an instability, wherein the material substantially inflates, generating large pores even though T < T-g. The mechanism for this instability is attributed to a small population of counterions, resulting from slight nonstoichiometry of polyelectrolytes, as well as the polymers themselves, a contribution estimated using Des Cloizeaux's theory of osmotic pressure for overlapping chains. Low concentrations of salt in the bathing solution are enough to overcome the osmotic pressure within the PEC, and it remains dimensionally stable over the long time periods studied. The universal practice of rinsing PECs, whether they are in macroscopic or thin-film morphology, in pure water should be re-evaluated.