Electrophilic aromatic substitution reactions were studied at polyp-xylylene) (PPX) film surface-reaction medium interfaces. The extent of the reactions (depth of penetration and degree of substitution) was determined by the interaction of the polymer with the reaction solution. Reaction with chlorosulfonic acid to produce sulfonyl chloride and sulfone functionalities occurred readily in the bulk of PPX, and yields were sensitive to time and temperature. Confinement of this reaction to the PPX surface was achieved by controlling the concentration of the acid. Functionalization of PPX with N-methylol-2-chloroacetamide in sulfuric acid to produce the chloroamidomethylated derivative occurred in high yield and was confined to the surface region of PPX. Hydrolysis of the amide to generate aminomethylated PPX was assessed by XPS and a derivatization reaction. Friedel-Crafts type chemistry (acylation and alkylation reactions) also produced functionalized surfaces, but with lower degrees of substitution than the other two reactions and was strictly surface-confined.