Flame synthesis of single walled carbon nanotubes (SWNT) is demonstrated using Fe nanoparticles [introduced by nebulization of an iron (III) nitrate salt solution] within a pyrolysis flame configuration. The roles of the nebulized solution solvent, metal nitrate concentration, pyrolysis flame gas composition, and the surrounding flame gas composition are interpreted as reflecting suitable concentrations of reactants without excessive pyrolysis products or deactivating species. The preferential reactivity of the catalyst particle toward CO and not toward C2H2 is understood as reflecting nanoparticle restructuring, interactions between co-adsorbates and reactant pyrolysis products. Fe is found to exhibit preferential reactivity toward CO for SWNT catalysis with this reactivity being strongly dependent upon catalyst particle size within our flame conditions. H-2 appears to moderate the dissociative adsorption through electronic interactions with co-adsorbates, mediated by the catalyst nanoparticle and by maintaining a catalytically clean particle surface.