The early stages of a gas phase pretreatment for diamond nucleation in an oxy-acetylene flame were investigated. The pretreatment involved a low oxygen-to-acetylene ratio (Rf = 0.93) performed at 15 mm from the torch on a scratched Si substrate. The nature of the carbon species deposited was analyzed using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and scanning electron microscopy. The pretreatment process produced diamond crystals after 30 seconds and a complete film in the center of the deposition area after 180 seconds. Furthermore, the pretreatment process kept the initial Siob2 layer less than the thickness generated by the control conditions, (Rf = 0.97, d = 10mm). The success of the pretreatment was attributed to changes in the flame structure and chemistry from the control conditions.