As　active　phases　in　low-temperature　Fischer-Tropsch　synthesis　for　liquid　fuel　production,　epsilon　iron　carbides　are　critically　important　industrial　materials.　However,　the　precise　atomic　structure　of　epsilon　iron　carbides　remains　unclear,　leading　to　a　half-century　of　debate　on　the　phase　assignment　of　the　epsilon-Fe2C　and　epsilon＇-Fe2.2C.　Here,　we　resolve　this　decades-long　question　by　a　combined　theoretical　and　experimental　investigation　to　assign　the　phases　unambiguously.　First,　we　have　investigated　the　equilibrium　structures　and　thermal　stabilities　of　epsilon-FexC　(x　=　1,　2,　2.2,　3,　4,　6,　8)　by　first-principles　calculations.　We　have　also　acquired　X-ray　diffraction　patterns　and　Mossbauer　spectra　for　these　epsilon　iron　carbides　and　compared　them　with　the　simulated　results.　These　analyses　indicate　that　the　unit　cell　of　epsilon-Fe2C　contains　only　one　type　of　chemical　environment　for　Fe　atoms,　while　epsilon＇-Fe2.2C　has　six　sets　of　chemically　distinct　Fe　atoms.