The consecutive phase transformations of a precipitated spray-dried iron-based catalyst for slurry Fischer-Tropsch synthesis (FTS) during activation and reaction process were investigated using Mossbauer effect spectroscopy (MES). It was found that the fresh iron catalyst activation in situ using syngas resulted in the formation of a mixture of iron carbides and superparamagnetic (spm) phases. The relatively small size of fresh iron crystallites was an important factor in the formation of epsilon'-Fe2.2C. During the reduction process, Fe3+ (spm) phase was easier to be reduced than alpha-Fe2O3 phase. Fe3O4 was not an active phase for FTS. The transformation of alpha-Fe2O3 into Fe3O4 before carbides formation was necessary to obtain FTS activity of the iron catalyst. There was a correlation between the content of CH4 in tail gas and the amount of iron carbides during activation. It was found that carbonization was the dominating phase transformation when the FTS reaction temperature increased from 250 C to 270 degrees C. However, the oxidization was more remarkably at higher FTS reaction temperature. chi-Fe5C2 was the main iron phase at lower reaction temperature. The changes in the bulk compositions resulted in the variation in catalyst activity during FTS. The results of this study showed that the active phase for FTS was a mixture of carbides and corresponding amounts of superparamagnetic phase. (C) 2008 Elsevier B.V. All rights reserved.