The mechanical properties of solution-cast films of filled polymers are found to be very much enhanced when the polymer is strongly adsorbed on the filler in the casting solution. Acid-base interactions between polymer and filler promote such adsorption, subject to the acidic or basic nature of the solvent. ^-Potential measurements were used as a measure of acid-base interaction between filler and polymer in solvents of low dielectric constant. Thin films containing up to 60 vqI % filler were cast from both acidic and basic solvents. Relative values of Young's modulus and tensile strength were compared for each system using the equations of Kerner and Nielsen as a basis for the cases of good and poor adhesion. In a system with acidic polymer (post-chlorinated PVC) and basic filler (CaC03 or BaTi03) cast from an acidic solvent (CH2Cl2), both adhesion and mechanical properties were improved over the same system cast from a basic solvent (THF); adsorption measurements showed no polymer adsorption from THF, but rapid and strong adsorption from CH2CI2. For the most part, in systems where both polymer and filler were either acidic or basic, brittle films (or none at all) were obtained.