The attachment of colloidal iron‐oxide nanoparticles (designated Fe‐NPs) to pristine and surface‐oxidized carbon nanotubes (CNTs and CNT‐Ox, respectively) was investigated. The loadings of Fe‐NPs (size 7 nm) on the CNT and CNT‐Ox supports amounted to 3.4 and 2.3 wt. %, respectively; the difference was attributed to weaker van der Waals interactions between the colloidal Fe‐NPs and the surface of CNT‐Ox. Fischer–Tropsch to olefins (FTO) synthesis was performed to investigate the impact of support functionalization on catalyst performance. Weak interactions between the Fe‐NPs and the CNT‐Ox support facilitated particle growth and led to substantial deactivation of the Fe/CNT‐Ox catalysts. The addition of promoters (Na+S) to Fe/CNT resulted in remarkable activity, selectivity to lower olefins, and stability, making colloidal iron nanoparticles on pristine CNTs a suitable catalyst for FTO synthesis.@en