The state and active site of iron-based catalysts in co-liquefaction of coal with low-density polyethylene (LDPE) have been discussed. The catalysts used were sulfur-promoted iron oxides (Fe2O3 + S), ferrous sulfide (FeS), ferrous sulfate (FeSO(4)(.)7H(2)O) and the mineral pyrite (FeS2). It was found by X-ray photoelectron spectrometry that the active site in the working state of Fe2O3 + S catalyst was not Fe1-xS and the main form of sulfur existing in the spent Fe2O3 + S catalyst was sulfate, followed by sulfite (SO32-). A finding from autoclave tests was that the ferrous sulfate before and after oxidation treatments showed sufficiently high activity for the co-liquefaction of coal with LDPE. It was concluded that an active site of the iron-based catalysts was sulfate species formed on the catalyst surface during the hydroliquefaction process of coal.