Carbon nanotubes grown by chemical vapor deposition have revealed differences in growth rate and characteristics depending on the underlying buffer layer. As determined by in situ photoelectron spectroscopy, iron catalyst deposited onto Si substrates covered with Al2O3, TiN, or TiO2 underwent different chemical changes during annealing and exposure to C2H2 as function of buffer layer. Formation of thin tubes (2-4 walls) and fast growth rate were associated with conversion of Fe onto FeO particles on Al2O3 layers. On the other hand, thick multiwalled tubes (more than 20 walls) grew from pure Fe particles formed onto TiN and TiO2 layers. The influence of different buffer layers is attributed to a combination of chemical and morphological changes induced in the catalyst due to catalyst-substrate interaction.