Single-walled carbon nanotube (SWNT) formation in a diffusion flame is analyzed for catalyst particle composition and SWNT growth rate. A typical fuel/air diffusion flame is sooty and not suitable for nanotube growth but by using a combination of oxygen-enrichment and fuel dilution, fuel oxidation is favored over pyrolysis and subsequent soot formation. Furthermore, an inverse flame configuration is used to create a flame environment that is favorable for SWNT growth. The optimum flame conditions and location for SWNT growth as well as the optimum catalyst particle composition were determined in situ using a scanning mobility particle sizer (SMPS) to measure the size distribution of the flame. The optimum catalyst particle composition was determined by comparing the size distributions of three flames with the same temperature but different levels of oxygen-enrichment. Increasing oxygen-enrichment oxidized the catalyst particles from iron to iron(II) oxide to iron(III) oxide with SWNTs being produced primarily from iron(II) oxide. Thus, oxygen-enrichment can be used to eliminate soot formation and to optimize catalyst particle composition. Finally, the SMPS and flame velocity measurements were used to measure growth rate. SWNTs were found to form in the post-flame region and grow at a maximum rate of over 100 μm/s.