Supercritical Fischer-Tropsch Synthesis (SC-FTS) using a potassium-promoted iron-based catalyst has been shown to produce large amounts of heavy (C10+) aldehydes and methyl ketones, while traditional gas phase FTS does not produce these compounds in significant amounts under either fixed or slurry bed operation. In order to better understand this behavior, a series of studies was undertaken to determine the effect of process conditions (H-2/CO ratio, temperature, pressure, and supercritical hexanes media ratio) on the performance of iron-based SC-FTS generally, and on aldehyde formation specifically. Over the range of process conditions studied, heavy aldehyde selectivity was found to decrease with increasing temperature, while both elevated pressure and increased media ratio favored aldehyde production. Changes in the H-2/CO ratio had little influence on syncrude functionality. The role of potassium promotion was also investigated by operating a potassium-free iron-based catalyst under SC-FTS conditions. In the absence of potassium promotion, no heavy aldehydes were detected.