The explanation for CH4 selectivity for iron based Fischer-Tropsch catalysts in the low conversion region (Le., < 50%) remains elusive. In this contribution, the CO conversion effect was carefully examined over four K promoted Fe catalysts (100 Fe/5.1Si/2Cu/ x K, where x = 1-5) over a wide range of CO conversion (i.e., 4-85%). Moreover, the effect of CO conversion on oxygenate selectivity of the Fe-K catalysts was carefully studied as well. The change in CH4 selectivity with CO conversion was found to resemble asymmetric "V" shaped curves, with the minimum values occurring at approximately 50% CO conversion. Adding greater than x = 2 K significantly alleviated the CO conversion effect which was attributed to the high K loading greatly decreasing the surface H coverage while improving CO adsorption. The unique CH4 selectivity trend suggests a complicated CH4 formation process that results from different aspects of the catalyst (i.e., chain growth and hydrogenation rates), and process conditions. Oxygenate selectivity was in the range of 0.7-2.8% and varied with the CO conversion and K loading. The addition of K up to x = 3 was found to promote oxygenate formation and chain growth. The overall oxygenate distribution up to C-17 follows an Anderson-Schulz Flory (ASF) distribution, with ethanol being the dominant oxygenate. Mechanisms of oxygenate formation different from that of hydrocarbon formation (e.g., CO insertion versus CO dissociation) were proposed to explain the experimental results.