Four iron catalysts (unpromoted, K-promoted, Si-promoted and K,Si-promoted) were activiated and subjected to common Fischer-Tropsch synthesis conditions. At increasing times on stream, samples were withdrawn from the continuously stirred tank reactor in the reactor wax while keeping the sample blanketed with an inert gas. Mossbauer spectra were recorded for various samples and the iron phases of the catalyst were compared to the catalytic activity. A simple model based on bulk composition of the catalyst is not related to the catalytic activity during the course of the run.