To investigate the influence of a supercritical fluid reaction medium on nanoparticle-catalyzed Fischer-Tropsch synthesis (FTS), Fe-based nanoparticle catalysts supported on carbon nanotubes were prepared and tested under both gas phase and supercritical FTS operating conditions. The nanoparticle catalysts displayed high activity, low CH4 selectivity, and elevated oxygenate selectivity. Conversion and selectivity towards heavy, un-hydrogenated products were elevated under supercritical operation. With K promotion, aldehydes were produced under both gas phase and supercritical conditions. This study demonstrated the necessity of K promotion for aldehyde production and demonstrated that appreciable amounts of aldehydes can be produced in gas phase, Fe-catalyzed FTS.