Polymeric　carbon　nitride　(PCN)　is　a　promising　class　of　materials　for　solar-to-chemical　energy　conversion.　The　increase　of　the　photocatalytic　activity　of　PCN　is　often　achieved　by　the　incorporation　of　heteroatoms,　whose　impact　on　the　electronic　structure　of　PCN　remains　poorly　explored.　This　work　reveals　that　the　local　electronic　structure　of　PCN　is　strongly　altered　by　doping　with　sulfur　and　iron　using　X-ray　absorption　spectroscopy　(XAS)　and　resonant　inelastic　X-ray　scattering　(RIXS).　From　XAS　at　the　carbon　and　nitrogen　K-edges,　sulfur　atoms　are　found　to　mostly　affect　carbon　atoms,　in　contrast　to　iron　doping　mostly　altering　nitrogen　sites.　In　RIXS　at　the　nitrogen　K-edge,　a　vibrational　progression,　affected　by　iron　doping,　is　evidenced,　which　is　attributed　to　a　vibronic　coupling　between　excited　electrons　in　nitrogen　atoms　and　C–N　stretching　modes　in　PCN　heterocycling　rings.　This　work　opens　new　perspectives　for　the　characterization　of　vibronic　coupling　in　polymeric　photocatalysts.　©　2020　The　Authors.　Small　Methods　published　by　Wiley-VCH　GmbH