SF6　has　been　designated　as　a　greenhouse　gas　by　the　Kyoto　Protocol　because　of　its　high　global　warming　potential.　C5-perfluorinated　ketone　(C5F10O　or　C5-PFK)　has　excellent　insulation　properties　and　environmental　characteristics.　and　has　been　chosen　as　an　alternative　gas　to　SF6.　Calculation　of　thermal　decomposition　is　usually　based　on　local　thermodynamic　equilibrium　(LTE)　and　local　chemical　equilibrium　assumptions.　However,　the　composition　may　not　reach　chemical　equilibrium,　particularly　at　lower　temperatures　and　high　rates　of　temperature　increase.　In　order　to　compare　the　chemical　non-equilibrium　and　local　thermodynamic　equilibrium　compositions,　a　chemical　kinetic　model　is　used　to　calculate　time　evolution　of　composition;　this　considers　the　effects　of　the　energy　barrier　in　chemical　reactions.　To　obtain　the　required　data　for　the　model,　which　is　necessary　because　of　the　limited　research　on　reaction　rates　in　C5-PFK　decomposition,　the　methods　of　computational　chemistry　are　used　to　calculate　transition　states　and　reaction　rates.　The　LTE　composition　is　calculated　using　Gibbs　free　energy　minimization.　Calculations　are　presented　for　two　possible　C5-PFK　thermal　decomposition　products,　C4F7O　and　C3F4O.　The　result　indicates　that　chemical　kinetics　is　required　to　calculate　the　dissociation　of　C5-PFK　at　low　temperatures,　such　as　occur　during　arc　ignition　and　near　the　arc　boundary.