The influence of solvents (water, ethanol/water mixtures) on the stability and mobility of polyelectrolyte multilayer (PEM)-supported lipid layers is investigated by the technique of fluorescence recovery after photobleaching (FRAP). Lateral lipid diffusion coefficients provide a measure of the interfacial interaction of lipid layer and polyelectrolyte multilayer cushion. An enhanced diffusion is found in solvent-swollen lipid layers in comparison to layers in air, with diffusion coefficients on the order of 10(-10) cm(2)/s. The effect of solvent is discussed in terms of solvent molecules screening the interaction of lipid and polyelectrolyte. The results of lateral diffusion in water-swollen PENI-supported lipid layers are interpreted in a model of headgroup-interaction dominated diffusion, whereas the chains are disordered and do not influence the mobility. Diffusion coefficients are governed by the nature of the lipid headgroup, and its respective electrostatic interactions: The ionic phosphatidic acid (DOPA) couples more strongly to the charged environment than the zwitterionic phosphatidylcholine (DOPC, DMPC). The above model explains the stronger binding of the PA headgroup in layers in water, whereas for layers in air the PC headgroup has a larger interaction. The distance dependence of the Coulomb versus dipolar interaction accounts for these results. Diffusion measurements are further performed in layers swollen in ethanol/water mixtures of varying composition. At an ethanol content below 50% the layers are stable and homogeneous. The extent of screening of the electrostatic interaction by solvent varies with its dielectric properties. At an ethanol content above 50% the state of PEM-supported lipid layers is inhomogeneous, which can be explained by either a partial dissolution of the lipid layer or a phase transition in the lipid layer. The findings oil charged polymeric systems complement investigations of uncharged polymer-supported model membranes, and demonstrate the significance of local electrostatic interactions for issues of stability and dynamics.