Thin films of polyelectrolyte complex were assembled using the multilayering method with direct, in situ observation of all multilayer components using attenuated total internal reflectance FTIR (ATR-FTIR). Buildup and ion doping of two representative combinations of positive and negative polyelectrolytes are controlled by salt type. The internal hydration of multilayers, measured precisely by ATR-FTIR, depends on the chemical identities of the polymers and the salt ions. The efficiency of doping inversely tracks the degree of hydration: less hydrated ("hydrophobic") ions are more efficient dopants, and less hydrated polyelectrolye complexes are harder to dope. Given that polyelectrolyte complexation is essentially entropy-driven, driving forces for doping, or association (the inverse of doping), are rationalized by counting all species in the condensed polyelectrolyte phase, including water molecules. For any combination of uniunivalent salt ions and polyelectrolyte, the strength of polyelectrolyte association is described by a single universal parameter. The magnitudes of the interactions per repeat unit are not high-a few kT-and are proportional to the number of water molecules released from the polymers when they form ion pairs. Hydration within multilayers due to residual salt is extensive but may be removed by an external osmotic stressing agent.