The pi-I-dependent complexation of poly(acrylic acid) (PAA) with phospholipid (phosphatidylcholine) vesicles was characterized by fluorescence polarization, differential scanning calorimetry (DSC), and surface pressure measurements of phospholipid monolayers. The complexation was pronounced below pH 4, when the polymer carboxyl groups are protonated, as shown by the binding of PAA to vesicles and the decrease in polymer mobility. The complexation was strong at low polymer concentrations and was weaker at higher polymer concentrations. PAA complexation increased the gel to liquid crystalline (LC) phase transition temperature (T-m) and enthalpy (Delta H) of the vesicles accompanied by a decrease in the transition cooperativity. This is most likely due to perturbations in the phospholipid headgroup upon PAA interaction. The effect of polymer adsorption on the phospholipid surface was investigated at the phospholipid/water interface, The ability of PAA to penetrate between the phospholipid molecules as a function of pH was determined by the lateral expansion of the monolayer at a constant surface pressure, The ability of the polymer to penetrate into the monolayer increased with decreasing pH. These results suggest that PAA complexation leads to expansion of the phospholipid packing of the vesicles by altering the phospholipid headgroup conformation.