The effect of potassium promoter loading (0, 0.5, 1.0 and 2.0 atomic ratio) on the performance of precipitated iron catalysts was investigated during Fischer-Tropsch synthesis using a continuously stirred tank reactor. Characterization by temperature-programmed reduction with CO, Mossbauer effect spectroscopy, and transmission/scanning transmission electron microscopy were used to study the effect of potassium promoter interactions on the carburization, phase transformation and carbon layer formation behavior of the catalysts. Under similar reaction conditions, all four catalysts exhibited similar initial CO conversions (similar to 85 %), whereas stability was found to increase with potassium loading up to 0.5 % (atomic ratio related to the iron), and further increases in potassium led to decreased activity. Unpromoted and excessively K loaded (2.0K/100Fe) catalysts exhibited similar deactivation trends with time and followed essentially similar conversion levels with time-on-stream. The selectivity of various potassium promoted catalysts was found to increase the average molecular weight of hydrocarbon products with increasing potassium loading. The deactivation rate was related to carbon deposition which could embed the iron carbide particles. If not enough K is present, Fe carbides tend to oxidize with TOS; with excessive K-loading, carbon deposition/site blocking become problematic.