A series of rich premixed flames are used to realize a post-flame gas mixture for optimum carbon nanotube (CNT) growth using inexpensive hydrocarbon fuels. The mixture of CO, CO2, H-2, and H2O is varied through use of hydrocarbon fuels with different H/C ratio in flames with different fuel/air ratios. Both SEM and HRTEM imaging are used to correlate the nanotube morphology and internal structure to the reaction gas composition. The variations observed are understood in light of the gas composition and the interaction of the reactive components with both the deposited Co catalyst particles and supporting metal substrate. Further comparisons between flames producing the same CO or H-2 Concentrations identify the roles of these gases in CNT synthesis. Optimal flame synthesis conditions, defined upon a H-2 and CO concentration map, are gauged on the basis of CNT length, relative surface density, and level of graphitic structure.