The lateral distribution of functional groups at polymer surfaces was studied with high resolution using atomic force microscopy with chemically modified tips ("chemical force microscopy"). Pull-off force measurements carried out on oxyfluorinated films of isotactic polypropylene as a function of pH using OH-functionalized tips revealed a "force pK(a)" of 5.5-6.0. At pH values close to or slightly higher than the "force pK(a)", position-resolved maps of pull-off forces showed laterally inhomogeneous adhesive forces on a sub-50 nm scale. These variations, which can be related to variations of local pK(a) values, are attributed to locally different hydrophobicity and thus to inhomogeneous distribution of the functional groups introduced by the surface treatment.