Chemical force microscopy (CFM) using probe tips that have been chemically functionalized to give hydroxylated (polar) or methylated (apolar) surfaces has been used to investigate frictional properties of polystyrene (PS) films that have been oxidatively modified to varying degrees using an ultraviolet-ozone (UVO) treatment. Oxygen chemisorption levels and functional group chemistry of the PS films have been followed using X-ray photoelectron spectroscopy (XPS), and the resulting data have been correlated with lateral (frictional) force measurements for each surface type. XPS analysis showed that UVO treatments introduced polar species at the film surfaces via the formation of C-O, C=O, and O-C=O functional groups. CFM was performed on the native and treated films using tips that were either hydroxyl- or methyl-terminated. The treated films exhibit substantially higher friction than untreated films when imaging with the hydroxyl-terminated tips. Friction is reduced when nonpolar methyl-functionalized tips are employed. The higher frictional forces and coefficients measured for the oxidized surfaces using the hydroxyl-terminated tip are due to hydrogen bonding between the polar hydroxyl groups of the tip and the oxygen functional groups of the PS surfaces.