Gold　nanoparticles　(GNPs)　sensitize　biomolecules　to　radiation　in　two　ways:　by　locally　increasing　the　radiation　energy　absorbed　and　by　modifying　the　sensitivity　of　the　target　biomolecules　to　radiation.　Taking　DNA　as　the　biological　target,　we　present　the　first　investigation　of　the　latter　chemical　mechanism　of　radiosensitization　by　irradiating　thin　films　made　of　GNP-DNA　complexes　with　10　eV　electrons.　Naked　GNPs　of　5　and　15　nm　diameters　were　synthesized　and　electrostatically　bound　to　DNA.　Damage　to　the　GNP-DNA　complexes　were　analyzed,　as　a　function　of　electron　fluence,　by　electrophoresis.　In　identical　5-monolayer　films,　the　yield　of　DNA　damage,　as　well　as　the　enhancement　factor　due　to　the　presence　of　5　nm　positively-charged　nanoparticles,　increased　with　rising　ratio　of　GNPs　to　DNA　up　to　1:1.　In　comparison,　increasing　the　ratio　of　negatively-charged　15　nm　GNPs　to　DNA　did　not　increase　damage.　As　verified　by　XPS　and　zeta　potential　measurements,　the　binding　of　plasmid　DNA　to　the　surface　of　the　two　sizes　of　GNPs　varies　owing　to　the　characteristics　of　the　GNP　surface　and　electrostatic　interaction.　The　results　indicate　that　strong　binding　of　GNPs　to　DNA　could　significantly　influence　the　efficiency　of　the　chemical　radiosensitization　mechanism.　This　mechanism　appears　to　be　an　important　component　of　the　overall　process　of　GNP　radiosensitization　and　should　be　considered　when　modeling　this　phenomenon.　Our　results　suggest　that　small　size　GNPs　(diam.　＜=　5　nm)　are　more　efficient　radiosensitizers　compared　to　larger　GNPs　when　delivered　into　cancerous　cells,　where　their　action　should　be　cell-cycle　dependent.